Physiology

Counts the actual number of artery-clogging particles rather than the cholesterol inside them. A better read on risk than LDL-C when the two disagree.

How much mineral is packed into bone. In men it is maintained mostly by ESTRADIOL rather than testosterone, which is why crushing oestrogen has a skeletal cost that takes years to show up.

The kidney's signal telling marrow to build red cells. Androgens raise it, and it is the upstream cause of the haematocrit rise rather than a separate effect.

The share of blood volume made of red cells. Thicker blood carries more oxygen and clots more easily, and this is the single most reliably moved number on cycle.

Oxygen-carrying protein in red cells. Moves with haematocrit and is the second half of the same reading.

The protective lipid fraction. Oral 17-alpha-alkylated steroids can cut it by half or more within weeks, and it is the fastest-moving marker of cardiovascular cost.

The fraction that deposits into vessel walls. Rises on most androgens and sharply on orals.

Tracks prostate activity and size. Useful mostly as a trend in men over forty rather than a single reading.

Beats per minute at rest. A cheap daily read on sympathetic tone, and usually the first number to move when something is pushing the nervous system.

Peak arterial pressure. Rises through water retention, thicker blood and vessel stiffening, and it is cheap to check at home weekly.

The carrier protein for thyroid hormone. When it falls, TOTAL T4 and T3 read low while the free, active hormone is unchanged, so the thyroid looks broken on paper when it is not.

Circulating fat. Moves with diet and insulin sensitivity as much as with any compound, which is why it needs reading alongside the rest of the panel.

Oil production plus blocked follicles, worst on back and shoulders. Tracks androgenic strength more than dose.

Hostility and short fuse. Reported consistently by users and in case series, and produced in blinded trials mainly at supraphysiological doses and mainly in a minority of people.

Restlessness, racing thoughts, a wired feeling. Multiple routes reach it: neurosteroid changes at GABA-A, glucocorticoid signalling, sleep loss and thicker blood.

Hunger drive. Some compounds raise it sharply and others kill it, and either can decide whether a diet phase works.

Plaque building in artery walls. The slow, silent, cumulative cost, driven by years of shifted lipids and pressure rather than by any single cycle.

Bile flow stalling in the liver. Itching, dark urine and jaundice, and a reason to stop rather than push through.

Blood clotting where it should not. Rises with haematocrit, and this is the reason a red cell count is worth watching rather than admiring.

Memory, focus and processing speed. It is the endpoint the nootropic peptides claim, and the one that is hardest to measure honestly outside a controlled trial.

Difficulty stopping despite harm, with low mood, lost drive and fatigue on withdrawal. Recognised as a genuine dependence syndrome in around a third of long-term users, not a character failure.

Low mood, flat drive and anhedonia. Concentrated in the withdrawal window after a cycle, when natural testosterone has not yet restarted.

Willingness to start things and to keep going. Reported separately from mood and separately from libido, and it moves separately: compounds exist that lift mood and flatten drive, and the reverse.

A narrowed range in both directions: less low, but also less warmth, less connection and less feeling for other people. Distinct from loss of pleasure, which is about reward specifically. Worth separating because the person experiencing it often rates it as an improvement while the people around them do not.

Depends on nitric oxide signalling, which needs DHT and adequate estradiol. Compounds that suppress natural testosterone without providing strong androgenic signal are the classic cause of failure here.

A lifted, expansive, everything-is-working feeling, arriving within days on some compounds. Recorded because it is real, because it is a large part of why people continue, and because it is the strongest single predictor of the dependence pattern further down. Not marked as good or bad: it is the reward that makes the rest of the list acceptable to the person having it.

Planning, switching between tasks, and holding back a response you are about to make. The last of those is the one that matters here: reported aggression on these compounds is often better described as a failure of the brake than as an increase in anger.

Total body fat. It is the endpoint the whole fat-loss class is measured on, and the one place where the incretin drugs have hard trial data rather than mechanism.

Freedom from pain, urgency and bloating. It is the outcome the gut-repair peptides are actually taken for, ahead of any measurable change to the lining.

Glandular breast tissue growth in men. Two separate routes reach it, estrogen and prolactin, and treating the wrong one does nothing.

Elevated mood, high energy, low sleep need, impaired judgement. The best-documented psychiatric effect of high-dose androgens, seen in a minority under blinded conditions.

Sperm count falling to subfertile or zero. Common on cycle, usually recovers over months to years, and occasionally does not.

Difficulty falling or staying asleep, often with night sweats. Its own problem, and an amplifier for every mood effect above it.

A lower threshold for annoyance, distinct from outright aggression and far more commonly reported.

How joints feel under load. Improved by fluid and collagen support, worsened by compounds that dry tissue out.

Muscle actually gained. The reason any of this is being taken.

Muscle lost alongside fat during rapid weight loss. Roughly a quarter to a third of the weight lost on a GLP-1 agonist without resistance training is lean tissue, which is why the newer dual agonists are engineered to spare it.

Thickened heart wall with reduced filling and relaxation. Documented in long-term users on imaging, and it does not fully reverse in everyone who stops.

Sexual desire. Needs androgen, a workable estradiol level and prolactin that is not elevated, so it fails from several directions at once.

Benign or malignant liver growths reported after prolonged high-dose oral steroid use. Rare, serious, and tied to years rather than weeks.

Things that used to be enjoyable stop landing. Distinct from low mood: a person can have normal mood and still get nothing out of food, sex, music or company. Reported most on the progestogenic and prolactin-raising compounds, and it is the effect people most often fail to name because it does not feel like sadness.

Permanent thinning at the crown and hairline in those genetically predisposed. Androgens accelerate a process that was already going to happen, and the loss does not come back.

Day-to-day mood and sense of wellbeing, as distinct from a diagnosable mood disorder. It moves in both directions on these compounds, and the direction is not predictable from how strong the compound is.

Fast shifts between states rather than a sustained one. A different claim from either raised or lowered mood, and it is the one most often reported by partners rather than by the person taking the compound.

The dose-limiting side effect of every incretin drug. It comes from the same delayed gastric emptying that produces the appetite drop, so it is not separable from the mechanism, only titratable.

Intense, unpleasant, memorable dreams, usually alongside broken sleep and night sweats. Named separately from insomnia because people report it even when total sleep time is normal, and because it is one of the few effects that reliably resolves within days of stopping.

Discrete episodes of overwhelming fear with a physical surge: pounding heart, breathlessness, a conviction that something is badly wrong. Different from sustained anxiety and worth its own entry because the plausible route is different too, running through sympathetic tone and a raised resting heart rate rather than through mood.

Suspicion of other people's motives that does not respond to evidence. Reported on the strongest compounds and named directly in the older user-facing notes on trenbolone. The evidence is case reports and self-report, and the self-report is unreliable in exactly the way the symptom predicts.

Blood-filled cavities in liver tissue. Rare, associated with long-term 17-alpha-alkylated use, and can bleed without warning.

How fast simple mental operations run. Included because it is the measure that moves with sleep debt, so it separates a genuine cognitive effect from an effect of being tired.

A larger prostate with weaker urine flow and night waking. Driven by DHT more than by testosterone itself.

Darkening of the skin, including existing moles. It is the intended effect of the melanotans and the reason dermatological review before and during use is not optional.

Breathing interruptions during sleep. Worsened by weight gain, fluid retention and neck mass, and it drives daytime blood pressure up further.

How restorative sleep is, as opposed to how much of it there is. Deep and REM sleep are where growth hormone is released and where memory is consolidated, so this is the measure that connects a sleep effect to a memory effect. A person can sleep eight hours and get almost none of this.

Mental rotation and navigation. The cognitive domain with the clearest sex difference and therefore the one most often tested in androgen studies. Effects reported are small and inconsistent.

Force produced. Rises faster than tendon and ligament can adapt, which is the risk built into the benefit.

Thoughts of ending your life, most often reported in the weeks after stopping rather than during use, when natural testosterone has not restarted and mood has not recovered. It is documented in case series and in withdrawal cohorts. It is listed here because leaving out the most serious outcome in the group would make everything above it read as the full picture.

A tendon tearing under load it should have handled. The classic pattern is a fast strength gain on a dry compound with connective tissue that has not caught up.

Shrinkage from lost LH signalling. Usually reversible, and the visible sign that the axis is switched off.

Recall of words, names and conversation. Studied in testosterone trials in older men with mixed and generally small results, which is worth stating plainly: this is one of the few cognitive measures here with any controlled human data at all, and the data does not show a large effect.

Male traits appearing in a female user: deeper voice, facial and body hair, clitoral growth. Voice change and clitoral growth are usually permanent, which makes this the one side effect that cannot be managed after the fact.

Fluid held under the skin and in tissue. Cosmetically a smoother look, clinically a blood pressure and a strain on the heart.

The cluster that arrives after stopping, while the natural axis is still shut down: flat mood, no drive, no libido, disturbed sleep, and in some cases suicidal thoughts. Separated from dependence because dependence is about the pull to continue and this is about what happens when you do not. It is the window in which almost all of the serious psychiatric events in the literature occur.

Holding and manipulating information over seconds. The cognitive measure most sensitive to lost sleep and to raised cortisol, which makes it the one most likely to move on these compounds and the one least likely to be noticed as a cognitive effect rather than as tiredness.

How many androgen receptors a tissue actually has. It is not fixed: training raises it, and sustained high-dose exposure lowers it in some tissues, which is part of why results flatten and doses creep upward.

Growing new blood vessels. It is the rate limit on healing: tissue cannot repair faster than it can be supplied, which is why so many healing peptides are described as pro-angiogenic.

Brain-derived neurotrophic factor acting at TrkB. The growth signal behind new synapses and behind the kind of memory that needs sleep to form. It is suppressed by sustained cortisol and by lost deep sleep, which is how a compound with no direct action in the brain still reaches memory.

Bile moving out of the liver and into the gut. The 17-alpha-alkyl group that lets an oral survive first pass is also what slows this, and the stall is what is felt as itching and jaundice.

Growth of the duct tissue behind the nipple, driven by oestrogen and progesterone signalling. Once it has laid down glandular tissue it does not reliably regress.

Cells moving to where the damage is. Wound closure depends on it, and it is how the actin-binding healing peptides are described as working.

Building the protein that tendon, ligament, joint surface and skin are made of. It moves slower than muscle does, and not every androgen supports it equally.

How strongly dopamine is signalling, mostly in the reward and motivation circuits. This is the route by which a compound that raises prolactin ends up flattening drive: dopamine holds prolactin down, so a sustained prolactin rise is read backwards as reduced dopaminergic tone.

Releasing stored fat for fuel. Androgen receptor density on fat cells is part of why some compounds visibly change body composition beyond their muscle effect.

The shortening of each hair’s growth phase until the follicle produces a thinner, shorter shaft. This is the actual mechanism of pattern hair loss, and it is gradual rather than sudden.

How fast the stomach passes food on. Incretin drugs slow it, which is what makes a smaller meal feel like enough, and also what causes the nausea and the fullness that will not clear.

Genes being read into protein. The androgen receptor is a transcription factor, so this is the actual first step after binding: everything else downstream is a consequence of which genes get switched on and in which tissue.

Moving glucose out of the blood and into muscle. It can happen through insulin or, independently of it, through the AMPK pathway that exercise uses.

Burning fuel as heat rather than storing it. Raising it increases energy expenditure without requiring more activity, which is the appeal of the glucagon component in the dual and triple agonists.

The stress axis: hypothalamus to pituitary to adrenal cortex, ending in cortisol. Relevant to mood twice over. A compound that blocks the glucocorticoid receptor removes the brake the axis runs on, and a compound that shuts down the gonadal axis leaves this one carrying more of the load.

How well tissue responds to insulin. Falling sensitivity means higher fasting insulin for the same blood sugar, and it is one of the quieter costs of long steroid use.

How much passes between gut cells rather than through them. Raised permeability lets partly digested material reach the immune system, which is the mechanism behind the leaky gut claim.

Pigment production in the skin. Driving it produces a tan without sun exposure, and it is the only proven effect of the melanotans.

Turning the day's experience into durable memory, which happens during deep and REM sleep rather than during the day. This is the step that breaks first when sleep breaks, and it is why memory complaints on these compounds usually trace back to sleep rather than to anything the compound does in the brain.

How efficiently cells turn fuel into usable energy. When it degrades the leak shows up as oxidative stress rather than as tiredness alone.

The rate at which muscle is built. Androgen receptor activation raises it and holds it raised, which is the entire point of the exercise.

The nuclei inside a muscle fibre, each running a patch of it. Androgens add them, and animal work shows the added ones persist for a long time after the drug is gone. This is the cell-level reason a past cycle is never entirely undone.

The body's own brake on muscle growth. Lowering it removes a ceiling rather than adding a push, which is a different kind of effect from raising protein synthesis.

Steroids made or converted inside the brain that act on ion channels rather than on nuclear receptors. The route by which an androgen changes anxiety and sleep without touching the androgen receptor.

Vesicles fusing with the nerve terminal to release their signal. The SNARE proteins do the fusing, and the cosmetic peptides that soften expression lines work by interfering with them.

Whether the body is holding more protein than it loses. A practical readout of net anabolism rather than a mechanism in its own right.

Damage from reactive oxygen outpacing the cell's ability to mop it up. One of the mechanisms behind androgen effects on liver and heart tissue that are not explained by the receptor alone.

Muscle being taken apart, driven largely by cortisol at the glucocorticoid receptor. Blocking it is the anti-catabolic half of how steroids work.

Red cell manufacture, driven by erythropoietin from the kidney. Androgens raise both the signal and the marrow's response to it.

HDL clearing cholesterol out of vessel walls and back to the liver. Oral steroids induce hepatic lipase, which dismantles this clean-up crew.

Dormant stem cells beside each muscle fibre. Androgens push them to divide and fuse into the fibre, which is where new myonuclei come from.

How much oil the skin’s glands put out. Androgens drive it up, and it is the step between receptor binding in the gland and a visible breakout.

Removing cells that have stopped dividing but refuse to die and keep releasing inflammatory signals. Senolytics are built to force that clearance.

How strongly serotonin is signalling. The system most often named when 19-nor androgens are linked to impulse control, irritability and sleep. Almost all of the evidence is rodent work; nothing has been measured at these receptors in a human on these compounds.

Holding onto salt and the water that follows it. Shows up as a smoother look, a heavier scale reading and a higher blood pressure.

Needs FSH plus a testosterone concentration inside the testis roughly a hundred times blood level. External steroid raises blood level and shuts the internal one down, which is why fertility goes first.

How hard the fight-or-flight side of the nervous system is running. Androgens raise tyrosine hydroxylase and change adrenergic receptor density, so a compound can lift this without touching mood directly. It is the shared cause behind a cluster people treat as four separate problems: a higher resting pulse, night sweats, a short fuse, and sleep that will not come.

The brain's ability to strengthen and rebuild connections. It is what learning physically is, and it is the endpoint almost every nootropic peptide claims.

Turning immature precursors into working T-cells in the thymus. It falls off with age, and it is what every thymic peptide claims to restore.

Preserving the protective caps on chromosome ends that shorten each time a cell divides. Claims of telomerase activation belong here, and human evidence for them is thin.

The heart wall thickening and stiffening in response to sustained load and to androgen signalling in cardiac muscle. It is measured on an echocardiogram, not felt.

The receptor myostatin uses to put the brakes on muscle growth. Every myostatin inhibitor works by keeping the ligand off this receptor rather than by acting on it directly.

Amylin is released alongside insulin and signals fullness through a different brain pathway than GLP-1. That is the whole argument for cagrilintide: two satiety routes at once rather than a bigger dose of one.

The receptor every anabolic-androgenic steroid is designed to hit. Binding moves it into the nucleus where it switches on genes for muscle protein synthesis, and in other tissues for hair loss, prostate growth and oil production.

The fat-cell adrenergic receptor. Activating it releases stored fat and raises heat production without the heart-rate effect of beta-1. It is the claimed target of the GH fragments that are supposed to burn fat without growing anything.

The other main dopamine receptor family, tied to reward learning and drive. Altered receptor density is one of the more reproducible findings in animal studies of nandrolone.

The dopamine receptor that holds prolactin down and carries reward and motivation signalling. Losing D2 tone raises prolactin and flattens drive and sexual response.

Drives epithelial cells to divide and migrate over a wound. It is one of the several targets BPC-157 is credited with, and it is also a well-known oncology target, which is the reason the growth-signal caution exists.

Where EPO tells the bone marrow to make red cells. Separating this from EPO's tissue-protective signalling is the entire design goal of ARA-290: repair without raising haematocrit.

The estrogen receptor that dominates breast tissue, liver and the pituitary feedback loop. Gynecomastia and much of the negative feedback on LH run through it.

The second estrogen receptor, common in brain, bone and vascular tissue. Part of why estradiol is needed for mood, libido and bone density rather than being purely a side effect to crush.

Fibroblast growth factor's receptor. In the brain, NCAM signals through it to grow new neurites, which is the route FGL is designed to take.

The receptor that tells inflammation to stop and start clearing up, rather than blocking it from starting. Humanin and LL-37 both act here.

The brain's main calming channel. Some androgen metabolites sit on it as neurosteroids and change anxiety and sleep quality without touching the androgen receptor at all.

What ghrelin binds to make the pituitary release growth hormone, and what every GHRP and MK-677 is built to hit. It also carries the hunger signal, which is why some GH secretagogues make you ravenous and others do not.

The other half of the GH release switch. GHRH sets how big a growth hormone pulse can be; the ghrelin receptor sets whether one fires. Hitting both at once is why a GHRH analog and a GHRP stack rather than duplicate.

The second incretin receptor. Adding it to GLP-1 agonism is what separates tirzepatide from semaglutide: more energy expenditure and a direct effect on fat tissue, on top of the appetite drop.

The incretin receptor behind semaglutide and every drug in that class. Three separate effects run through it: appetite suppression in the brain, slower stomach emptying, and glucose-dependent insulin release from the pancreas.

Glucagon's receptor. Agonising it raises energy expenditure and burns liver fat, which is why the triple agonists add it. It also raises blood glucose, which is the tension every dual and triple agonist has to balance.

The cortisol receptor. Some androgens bind it too, and whether they act as agonist or blocker at a given tissue is a large part of why compounds with similar AR affinity feel so different.

The pituitary receptor that decides whether LH and FSH are released. It only responds to PULSES. A steady signal desensitises it and shuts the axis down completely, which is why the same drug can restart testicular function or chemically castrate depending on how it is dosed.

The shared signalling subunit a large family of inflammatory cytokines runs through, IL-6 included. Humanin's cytoprotective effect is described as working through this complex.

Where growth hormone itself acts. Binding pairs the receptor and fires JAK2/STAT5, which in the liver means IGF-1 output and in fat means lipolysis. Injected GH works here; secretagogues work one step upstream.

Hepatocyte growth factor's receptor. In the brain it drives new synapse formation, which is the mechanism claimed for dihexa.

The receptor that carries almost all of growth hormone's anabolic effect, one step downstream of it. It is also the reason the IGF-1 analogs are the peptides with the most serious theoretical cancer question: the same signal that grows muscle grows anything already growing.

One step above GnRH. Kisspeptin neurons are the master switch that tells the hypothalamus to release GnRH at all, which is why acting here produces a physiologic LH pulse rather than overriding the axis.

The pigment receptor on melanocytes. Activating it makes the skin produce eumelanin, which is the tanning effect of the melanotans. The same receptor family also damps inflammation, which is what KPV uses it for.

A central melanocortin receptor involved in energy balance and sexual arousal. It is one of the two receptors PT-141 works through.

The melanocortin receptor that controls appetite and central sexual arousal. It is why the melanotans suppress hunger and cause spontaneous erections, and why PT-141 works in the brain rather than in the blood vessels the way a PDE5 inhibitor does.

The melanocortin receptor on sebaceous and other exocrine glands. Pan-melanocortin agonists hit it, which is part of why they can trigger oil production and acne alongside the tan.

The aldosterone receptor in the kidney. When a steroid activates it, sodium and water are held onto, which shows up as blood pressure and smooth-over-the-muscle bloat.

Oxytocin's receptor, and it is not only in the brain. The uterus, heart, gut and T-cells all carry it, which is why an intranasal dose taken for mood also has cardiovascular and gut effects.

The progesterone receptor. 19-nor steroids are structurally close enough to progesterone to activate it, which is why nandrolone and trenbolone produce progestogenic side effects that an aromatase inhibitor cannot touch.

What prolactin acts through, including in breast tissue. It is the reason a progestogenic compound can produce gyno with a perfectly normal estradiol reading.

A serotonin receptor involved in mood, impulse control and aggression. Androgen exposure shifts serotonergic signalling in animal work, which is the mechanistic case behind steroid irritability.

Part of how the immune system recognises a bacterial surface. Thymosin alpha-1 acts through it and TLR9 to push dendritic cells and T-cells toward a coordinated response.

Recognises bacterial and viral DNA inside the cell. The second of the two toll-like receptors thymosin alpha-1 is described as working through.

Nerve growth factor's receptor. It keeps sensory and cholinergic neurons alive and drives neurite outgrowth, which is what the porcine brain-extract peptides are claimed to mimic.

What BDNF binds. It is the main receptor behind the claim that a nootropic peptide improves learning or protects neurons: almost all of them are described as raising BDNF, which means acting here.

The main receptor for new blood vessel growth. Healing peptides that speed up tendon and gut repair are described as acting here, because tissue cannot repair faster than it can be supplied.

VIP's two receptors. They open airways, widen vessels, move the gut and damp cytokine output, which is why one peptide is studied for conditions as unrelated as pulmonary hypertension and inflammatory bowel disease.

Stores fat and houses most of the body's aromatase, so more fat mass means more conversion of androgen to estrogen.

Makes cortisol and the adrenal androgens. It answers to ACTH from the pituitary rather than to LH, which is why shutting down the testicular axis leaves it running.

The threat detector. Dense in androgen receptors, and the structure most implicated in the anger, vigilance and fear side of what these compounds do. Splitting it out of the limbic system matters because threat processing and reward processing sit in the same blob otherwise, and compounds pull them in opposite directions.

The gland that actually releases growth hormone, LH and FSH. Every GH secretagogue and every GnRH drug is aimed here, not at the tissue where the effect is finally felt.

Where red cells are built. Androgens push it harder, which is why they were once a treatment for anaemia and why haematocrit climbs on cycle.

Responds to both estrogen and prolactin. Once glandular tissue forms it does not go away on its own, which is why this one is worth preventing rather than treating.

The lining of the airways. It is the target of the lung bioregulators and one of the tissues VIP acts on.

Joint surface tissue with almost no blood supply, which is why it repairs so slowly and why it is targeted separately from tendon.

The single cell layer separating gut contents from the bloodstream. Most peptides sold for gut health are aimed at either repairing it or keeping its tight junctions shut.

Carries androgen receptors like skeletal muscle does, and thickens under the same signal. Unlike a bicep, a thicker left ventricle fills less well.

Where new memory is laid down. Unusually sensitive to sustained cortisol and to lost deep sleep, both of which these compounds can produce, which makes it the structure behind most reported memory complaints.

The control room above the pituitary. Appetite, body temperature, the reproductive axis and the GH rhythm are all set here, which is why so many peptides that produce whole-body effects act on one small area.

Controls sodium and water balance and produces erythropoietin, so it sits behind both the bloat and the rising red cell count.

The testicular cells that actually make testosterone in response to LH. Anything that restarts the axis has to reach these; anything that shuts the axis down starves them.

The emotional core of the brain, and androgen-receptor rich. This is where mood, threat response and aggression are set, and where steroid mental effects are thought to land.

Processes every oral steroid on first pass. The 17-alpha-alkyl group that lets an oral survive that trip is also what makes it hard on this organ.

The insulin and glucagon-producing cells. Incretin drugs raise insulin here only when glucose is already high, which is why they do not cause hypoglycaemia on their own the way injected insulin does.

The gland that makes melatonin and sets the day-night rhythm. The Russian bioregulator peptides are aimed at it on the theory that restoring that rhythm slows other ageing processes.

Planning, judgement and the brake on impulse. Relevant here because the reported behaviour change on these compounds is often better described as a weaker brake than as a stronger drive, and those two have different consequences.

Growth here is driven mainly by DHT rather than testosterone, which is why compounds that resist 5-alpha-reductase are gentler on it.

In a genetically susceptible scalp, DHT binding shrinks the follicle a little with each cycle. The predisposition is inherited; the androgen sets the pace.

Oil glands in the skin. Androgen signalling raises output, and heavier oil plus keratin plugging is how steroid acne starts.

The intended target. Dense in androgen receptors, and the tissue where the whole risk trade is being made.

The largest organ, and the target for the melanocortin and copper peptides. Pigment, collagen and wound repair all happen here.

Where ghrelin is produced and where GLP-1 agonists slow emptying. That slowing is both the satiety mechanism and the source of the nausea.

Connective tissue adapts far slower than muscle. A strength jump that outpaces it is the setup for a tear.

Makes testosterone and sperm, and needs LH and FSH from the pituitary to do either. Cut that signal and it shrinks and goes quiet.

Where T-cells mature. It shrinks steadily with age, which is the premise behind every thymic peptide: restore the training ground rather than the cells.

The single-cell lining of every blood vessel. It sets vessel tone and is where plaque begins when lipids and pressure both move the wrong way.