How Testosterone Influences Other Hormones, and What Changes During TRT

Nelson Vergel · 2026-11-10T11:25:46-0600

hormones and organs in the human body.webp

Testosterone affects far more than sex drive and muscle. Its activity intersects with reproductive hormones, estrogen, thyroid function, blood sugar regulation, growth hormone, and the signals controlling red blood cell production.

These relationships differ. Testosterone converts directly into certain hormones. Other hormones respond through feedback loops. Some changes occur indirectly through shifts in body fat, muscle, sleep, or metabolism.

Testosterone replacement therapy, or TRT, changes several of these relationships. Raising testosterone in your bloodstream also reduces your brain’s stimulation of your testes. Estradiol and dihydrotestosterone often rise, while luteinizing hormone and follicle-stimulating hormone fall.

The central distinction: TRT replaces testosterone exposure, but does not reproduce every function of a normally functioning reproductive system.

The hormone charts: a useful starting point with several corrections

The supplied images show the range of organs involved in hormone production. The first contains several errors requiring correction:

Growth hormone comes from the anterior pituitary.
LH stimulates testosterone production in testicular Leydig cells. FSH supports sperm production through Sertoli cells.
T3 is triiodothyronine. T4 is thyroxine. Both are thyroid hormones.
Cortisol is already a glucocorticoid.
Oxytocin and vasopressin originate in the hypothalamus and enter circulation through the posterior pituitary.

The second image offers a broader overview, but leaves out GnRH, the hypothalamic hormone directing reproductive hormone release. Neither image establishes how TRT affects each pathway. Those effects require evidence from human physiology and clinical studies.

Testosterone’s major relationships at a glance

Hormone or system	Relationship with testosterone	Typical effect of TRT	Evidence
GnRH, LH, and FSH	Regulate testicular testosterone and sperm production	Suppression through negative feedback	Established
Estradiol	Produced from testosterone through aromatase	Often rises	Established
DHT	Produced from testosterone through 5-alpha-reductase	Often rises, with differences among formulations	Established
Inhibin B	Reflects Sertoli cell activity and helps regulate FSH	Variable changes alongside suppression of sperm production	Context dependent
Prolactin	Excess prolactin suppresses reproductive signaling	No dependable corrective effect	Established clinical distinction
Thyroid hormones	Thyroid status affects testosterone binding and interpretation	Total thyroid hormone levels sometimes change through binding proteins	Established
Cortisol	Excess glucocorticoid exposure suppresses reproductive function	No consistent reduction in cortisol	Limited, variable treatment effects
DHEA and steroid precursors	Share steroid production pathways but differ in regulation	Selected precursors change, without universal adrenal shutdown	Limited human evidence
GH and IGF-1	Interact with sex steroids in growth and body composition	Increases occur in some studies	Variable
Insulin	Interacts with body composition and metabolic health	Insulin sensitivity sometimes improves, diabetes outcomes are inconsistent	Mixed
Leptin and gut hormones	Link nutrition, fat stores, appetite, and reproduction	Leptin often falls, other changes are inconsistent	Limited to moderate
Erythropoietin and hepcidin	Regulate red blood cell production and iron availability	Greater stimulation of red blood cell production	Established

1. LH, FSH, and GnRH: why TRT suppresses your own production

Your hypothalamus releases gonadotropin-releasing hormone, or GnRH, in pulses. GnRH stimulates the anterior pituitary to release LH and FSH.

LH tells the Leydig cells in your testes to produce testosterone. FSH works with high local testosterone concentrations to support sperm production.

Testosterone and estradiol provide negative feedback to the hypothalamus and pituitary. As hormone exposure increases, stimulation decreases.

External testosterone activates this same feedback system. Your bloodstream receives testosterone from treatment, while your pituitary reduces LH and FSH output. Your testes then receive less stimulation.

This explains an apparent contradiction: a man taking TRT might have normal or high blood testosterone while testosterone inside his testes falls sharply.

In a short randomized study of 29 healthy men, testosterone injections without hCG reduced intratesticular testosterone by 94%. Adding hCG preserved intratesticular testosterone in a dose-dependent fashion. The study lasted three weeks and evaluated local testosterone, so the findings do not establish long-term fertility preservation. [1]

For patients, the consequences include reduced sperm production and sometimes smaller testicular volume. Suppressed LH and FSH during TRT usually reflect expected feedback, rather than new pituitary damage.

TRT should not serve as contraception. Suppression varies, and some men continue producing sperm. Men planning conception need a fertility discussion before treatment. [2]

2. Estradiol: testosterone’s essential partner

The enzyme aromatase converts testosterone into estradiol. This conversion occurs in several tissues, including fat tissue.

Estradiol serves essential functions in men. Bone health, body fat regulation, and sexual function depend partly on adequate estrogen signaling.

A controlled human study separated testosterone effects from estradiol effects by suppressing natural hormone production, administering different testosterone doses, and blocking aromatase in one group. Testosterone deficiency primarily affected lean mass, muscle size, and strength. Estrogen deficiency primarily increased body fat. Both contributed to impaired sexual function. [3]

During TRT, more testosterone becomes available for conversion. Estradiol often rises along with testosterone. The degree varies with testosterone exposure, aromatase activity, body composition, and individual physiology.

An estradiol increase does not automatically require treatment. Breast tenderness or gynecomastia warrants evaluation, but fatigue, fluid retention, and reduced libido are not specific evidence of excess estradiol.

Lowering estradiol aggressively also carries consequences. Aromatase inhibition in the controlled study increased body fat and worsened aspects of sexual function despite testosterone administration. [3]

The practical goal is adequate testosterone and estrogen activity, assessed through symptoms and appropriate testing. A single estradiol result or testosterone-to-estradiol ratio should not dictate an automatic anastrozole prescription.

3. DHT: a separate conversion pathway

The enzyme 5-alpha-reductase converts testosterone into dihydrotestosterone, or DHT. DHT activates androgen receptors and has substantial effects in androgen-sensitive tissues.

TRT supplies additional substrate for this conversion. DHT often rises, but the response differs among formulations. Transdermal testosterone sometimes produces comparatively higher DHT exposure because conversion occurs in skin. The clinical importance of an isolated serum increase is uncertain. [2]

DHT differs from estradiol in one essential respect: DHT does not aromatize into estrogen.

For readers evaluating hair loss, prostate symptoms, or a DHT result, blood concentration alone gives an incomplete picture. Tissue sensitivity and local metabolism also matter. Lowering DHT is a separate treatment decision, rather than a routine requirement of TRT.

4. Inhibin B: why testosterone testing does not measure fertility

Sertoli cells produce inhibin B, which contributes to feedback control of FSH. This relationship connects pituitary signaling with testicular sperm-producing activity.

During TRT, lower FSH and lower intratesticular testosterone alter the environment supporting sperm production. Inhibin B responses vary, so an individual result does not reliably establish fertility.

A normal blood testosterone concentration during treatment does not confirm normal sperm production. For a man concerned about fertility, semen analysis addresses the practical question more directly than testosterone alone.

hCG acts at the LH receptor and stimulates testicular steroid production. FSH provides a different signal. Preserving intratesticular testosterone with hCG therefore does not guarantee normal sperm production in every patient. The short hCG experiment described above supports a mechanism, rather than a universal fertility protocol. [1]

5. Prolactin: a potential cause of low testosterone

With prolactin, the most important relationship often runs toward testosterone. Persistently elevated prolactin suppresses reproductive signaling and sometimes contributes to low testosterone, reduced libido, and infertility.

TRT raises testosterone exposure but does not treat the underlying cause of elevated prolactin.

The American Urological Association recommends prolactin testing when low testosterone occurs with low or low-normal LH. Persistent abnormalities require further evaluation. Breast symptoms also support measuring estradiol before TRT. [4]

This distinction matters when a patient’s testosterone improves but sexual symptoms persist. Raising the dose without investigating the original endocrine problem risks overlooking a treatable cause.

A prolactin result also requires context. Medication effects and temporary elevations differ from a persistent abnormality associated with pituitary disease.

6. Thyroid hormones: a change in binding is not necessarily thyroid disease

Thyroid hormones regulate energy use and influence the interpretation of testosterone results. Thyroid status affects sex hormone-binding globulin, or SHBG, which carries testosterone in blood.

SHBG is a transport protein, rather than a hormone. Changes in SHBG alter the relationship between total and free testosterone. This is one reason total testosterone alone sometimes gives an incomplete assessment. [2]

Testosterone also affects another transport protein: thyroxine-binding globulin, or TBG.

Androgen treatment sometimes lowers TBG and total T4 while free thyroid hormone concentrations remain unchanged. Testosterone prescribing information explicitly describes this effect. A lower total T4 during TRT therefore does not, by itself, establish hypothyroidism. [5]

When thyroid disease is suspected, TSH and free T4 provide more useful context. In patients with pituitary disease, TSH alone is insufficient to assess central hypothyroidism. [6]

TRT does not replace thyroid hormone treatment. Persistent fatigue, cold intolerance, or other symptoms deserve evaluation beyond testosterone dose adjustment.

7. Cortisol and ACTH: TRT does not reliably lower stress hormones

The hypothalamus and pituitary regulate adrenal cortisol production through CRH and ACTH. This system differs from the GnRH-LH-FSH pathway.

Excess glucocorticoid exposure, including certain medications, sometimes suppresses reproductive function. This does not imply a simple inverse relationship where every testosterone increase produces a cortisol decrease.

In a randomized trial of older men, 26 weeks of low-dose testosterone increased free testosterone but did not significantly change overnight cortisol secretion. [7]

Feeling less stressed after starting TRT does not establish lower cortisol. Improvements in energy, sexual function, or daily functioning offer other explanations.

Likewise, fatigue during treatment does not establish cortisol deficiency. A suspected adrenal disorder requires its own evaluation. Testosterone treatment is not a substitute for adrenal hormone replacement.

8. DHEA, pregnenolone, and progesterone: does TRT shut down every steroid pathway?

Testosterone, cortisol, DHEA, progesterone, and related steroids share cholesterol-based production pathways. Their shared origin does not mean one hormone controls production throughout the body.

Different tissues use different enzymes and regulatory signals. Testicular steroid production depends heavily on LH. Adrenal steroid production follows different regulation.

TRT suppresses LH, so selected testicular steroid precursors decline. A study of 25 men with congenital hypogonadotropic hypogonadism compared testosterone treatment with hCG/FSH treatment and healthy controls. During testosterone treatment, progesterone and 17-hydroxyprogesterone were lower than in controls. DHEAS and several other measured steroids were similar to controls. [8]

These findings argue against the claim “TRT shuts down all upstream hormones.” They also have limits. The participants had a specific congenital condition, and the study did not establish whether supplementing every altered precursor improves health or symptoms.

The clinical significance of selected precursor changes is uncertain. I cannot confirm this with high certainty: routine pregnenolone, progesterone, or DHEA supplementation improves outcomes for men receiving TRT. The unresolved questions include who benefits, which symptoms respond, and what long-term risks accompany treatment.

A shared biochemical pathway is not enough evidence to prescribe a supplement.

9. Growth hormone and IGF-1: interaction without a predictable response

Growth hormone, or GH, comes from the anterior pituitary. GH stimulates IGF-1 production, including production in the liver.

Testosterone and estradiol interact with this system. Some testosterone trials show increased GH secretion and IGF-1.

In a randomized study of 34 older men, 26 weeks of testosterone increased overnight GH secretion and raised IGF-1 by approximately 22% compared with placebo. This was a small study in a selected population, rather than a predictable response for every TRT patient. [9]

Another small randomized study comparing testosterone, an aromatase inhibitor, and placebo suggested estrogen conversion contributes to the IGF-1 response. [10]

TRT therefore influences the GH/IGF-1 system in some men, but does not reliably correct GH deficiency. Nor does a low-normal IGF-1 automatically justify GH or a GH-releasing drug.

Clinical benefit also requires more than moving a laboratory result upward. Changes in strength, function, symptoms, and adverse effects matter.

10. Insulin and glucose regulation: body composition changes do not guarantee better diabetes control

Insulin helps regulate blood glucose and nutrient storage. Low testosterone, obesity, and insulin resistance often coexist.

TRT frequently increases lean mass and reduces fat mass. These changes offer a plausible route toward improved insulin sensitivity, but improved body composition does not guarantee lower glucose or reduced diabetes risk.

The TRAVERSE diabetes substudy found no significant reduction in progression from prediabetes to diabetes with testosterone gel. Glycemic control also did not improve compared with placebo among participants with prediabetes or diabetes. [11]

This is an important distinction between mechanism and clinical outcome. A treatment might improve one metabolic feature without producing a measurable benefit in diabetes control.

Individual responses still vary. Testosterone labeling notes changes in insulin sensitivity and possible reductions in insulin requirements. Patients using insulin should follow glucose readings and coordinate medication adjustments with their clinician. [5]

TRT should address an appropriate testosterone deficiency diagnosis. Diabetes treatment needs its own monitoring and management.

11. Leptin, adiponectin, ghrelin, and other appetite signals

Fat tissue produces hormones, including leptin and adiponectin. The stomach and intestines produce signals involved in appetite and nutrient handling, including ghrelin, GLP-1, and PYY.

Changes during TRT reflect both direct hormone effects and changes in body composition.

In a randomized trial of 100 men with obesity undergoing a reduced-calorie diet, testosterone lowered leptin compared with placebo. Changes in adiponectin and measured gut hormones generally did not differ between groups. [12]

Other trials have reported reductions in adiponectin during testosterone treatment. This variation shows why “TRT improves every metabolic hormone” overstates the evidence. [13]

For practical follow-up, appetite, waist size, body composition, glucose control, and physical function offer more useful information than assuming a specific leptin or ghrelin response.

TRT does not reproduce the pharmacologic effects of a GLP-1 receptor agonist.

12. Erythropoietin and hepcidin: the hormonal explanation for rising hematocrit

One of testosterone’s clearest effects outside reproduction involves red blood cell production.

The kidneys produce erythropoietin, or EPO, which stimulates red blood cell formation. The liver produces hepcidin, which regulates iron availability.

Testosterone treatment stimulates this system through changes including increased EPO signaling and reduced hepcidin. More iron becomes available for red blood cell production.

A randomized trial in older men documented increased hemoglobin and hematocrit alongside early increases in EPO and suppression of hepcidin. The researchers proposed a change in the relationship between EPO and hemoglobin during testosterone treatment. [14]

This pathway helps explain both improvement in anemia and excessive hematocrit elevation in some patients.

Hematocrit monitoring addresses the treatment consequence. Routine EPO or hepcidin testing is generally unnecessary for standard TRT follow-up. A hematocrit above 54% warrants treatment interruption or adjustment and evaluation of contributing factors under clinical guidance. [2]

13. Aldosterone, renin, vasopressin, and fluid balance

Several hormones regulate blood pressure and fluid balance. These include renin, angiotensin II, aldosterone, vasopressin, and atrial natriuretic peptide.

Testosterone treatment sometimes causes sodium and water retention. This does not establish a predictable rise in any single fluid-regulating hormone.

The clinically established concern is blood pressure. In February 2025, the FDA announced class-wide labeling changes after ambulatory blood pressure studies confirmed increased blood pressure with testosterone products. [15]

Swelling or rising blood pressure during TRT should prompt assessment of the treatment regimen, medications, and other contributing conditions. Assuming estradiol caused every episode of fluid retention risks missing the relevant problem.

Blood pressure readings are more actionable than ordering a broad renin, aldosterone, and vasopressin panel without a specific indication.

14. PTH, vitamin D, and bone metabolism

Parathyroid hormone, or PTH, and active vitamin D regulate calcium and bone metabolism. Testosterone and estradiol contribute through separate, interacting effects on bone.

TRT does not replace vitamin D, calcium management, or treatment of a parathyroid disorder.

Bone density and fracture prevention also require separate interpretation. In the TRAVERSE fracture substudy, clinical fractures occurred in 3.50% of testosterone-treated participants and 2.46% of placebo-treated participants over a median follow-up of 3.19 years. Testosterone did not reduce fractures in this population. [16]

The study does not explain every possible mechanism behind the difference. Its practical message is narrower: improved testosterone exposure should not be assumed to provide adequate fracture protection.

Men with osteoporosis or fragility fractures need a dedicated bone-health assessment.

15. Melatonin, oxytocin, thymic hormones, and less-studied relationships

The images include hormones associated with sleep, social behavior, immunity, digestion, and other functions. Their presence in an endocrine chart does not establish a clinically meaningful response to TRT.

For melatonin, oxytocin, thymic hormones, gastrin, calcitonin, and several other signals, I cannot confirm this with high certainty: standard TRT produces a consistent change with a proven treatment benefit.

The missing information includes reproducible effects in typical hypogonadal men, the size of those effects, and whether targeted testing improves patient outcomes.

A patient feeling better after TRT does not establish which secondary hormone changed. Likewise, persistent symptoms do not prove a deficiency in one of these less-studied hormones.

Using these relationships in clinical follow-up

A practical assessment starts with the symptom or risk requiring attention.

Clinical question	Useful assessment
Is testosterone exposure appropriate?	Testosterone measured at the appropriate time for the formulation, with free testosterone assessment when indicated
Why was testosterone low before treatment?	Pretreatment LH and FSH, with prolactin and further evaluation when indicated
Is fertility being preserved?	Reproductive history and semen analysis
Are breast symptoms developing?	Clinical examination and estradiol testing
Is red blood cell production excessive?	CBC, hemoglobin, and hematocrit
Has blood pressure increased?	Repeated office or home blood pressure readings
Does fatigue suggest thyroid disease?	TSH and free T4 in the appropriate clinical setting
Is metabolic health improving?	Glucose or HbA1c, waist measurement, weight, and functional progress
Is fracture risk elevated?	Bone-health assessment and bone density testing when indicated