How Your Thyroid Controls Your Testosterone (Check This Before TRT)

Your thyroid is upstream of your testosterone. That sentence sounds simple, and it is, but the implications of it are why men stay on TRT for years when they never needed it in the first place.

Your body runs three major hormonal systems and they all flow through the same control center. You have a region of brain tissue called the hypothalamus, and sitting just below it is the pituitary gland. Together they function like a relay station for your entire endocrine system, and they manage your thyroid axis, your testosterone axis, and your growth hormone axis through the same infrastructure. Each axis follows the same basic logic: the brain sends a signal down to a gland, the gland produces a hormone, and that hormone feeds back up to the brain to keep the system in balance. Because they share that relay, what happens in one axis affects the others.

The thyroid axis runs first, and it runs deeper than most people appreciate.

Your hypothalamus produces something called TRH, which is thyrotropin releasing hormone, and it travels down to the pituitary and triggers the release of TSH, or thyroid stimulating hormone. TSH then travels to your thyroid gland and tells it to produce thyroid hormones. Your thyroid puts out two of them: T4, called thyroxine, which makes up about 80 percent of total thyroid output, and T3, called triiodothyronine, which makes up the remaining 20 percent. T4 is the storage form. T3 is the active form. And T4 has to be converted into T3 by enzymes called deiodinases before your cells can actually use it.

Think of T4 as raw material sitting in a warehouse and the deiodinase enzymes as the workers that process it into finished product. If you do not have enough workers, material piles up and finished product runs low. Those enzymes require selenium to function, which is why selenium deficiency directly impairs T4 to T3 conversion. Research by Winther and colleagues found that selenium deficiency is associated with a high free T4 to T3 ratio, meaning the raw material is there but it is not being processed.

There is also a diversion pathway where T4 gets converted into something called reverse T3 instead of active T3. Reverse T3 is biologically inactive and it can actually occupy T3 receptors without activating them, so it acts like a placeholder that blocks the real signal. This matters enormously when you are interpreting bloodwork, because your TSH can look completely normal while your active T3 is low and your reverse T3 is elevated. That pattern is called functional hypothyroidism and standard thyroid testing, which only checks TSH, misses it entirely.

Now here is where this connects to testosterone.

Your testosterone axis runs through the same relay. Your hypothalamus produces GnRH, which travels to the pituitary and triggers the release of LH and FSH. LH then travels to your testes and tells your Leydig cells to produce testosterone. The relay is identical. And because both axes share the same hypothalamus and pituitary infrastructure, thyroid hormone directly shapes how well the testosterone axis functions, through three separate mechanisms.

The first mechanism operates at the pituitary. T3 modulates how well your pituitary responds to the GnRH signal coming down from your hypothalamus. In hypothyroidism, the pituitary receives that GnRH signal but produces an inadequate LH response, so the signal arrives but the relay station does not pass it along with enough strength. This creates a pattern called hypogonadotropic hypogonadism, which is low testosterone with low or normal LH. The testes are fine. They are just not getting the signal to do their job. A 2000 study by Donnelly and White looked at men with primary hypothyroidism and found that their free testosterone nearly doubled after starting thyroxine replacement, rising from 161 to 315 pmol/L, and the pattern was hypogonadotropic throughout, meaning the problem was coming from the brain axis, not from testicular failure.

The second mechanism operates directly at the Leydig cell. Your Leydig cells have thyroid hormone receptors on them, and T3 does two things at that level. It increases the number of LH receptors on those cells, making them more responsive to whatever LH signal they receive, and it upregulates something called StAR protein, which stands for steroidogenic acute regulatory protein. StAR is the rate limiting step in testosterone production because it is the transporter that moves cholesterol into the mitochondria where steroidogenesis actually begins. Without enough cholesterol moving into that system, testosterone production is capped regardless of how much LH is arriving. Maran and colleagues showed that T3 induced a 260 percent increase in StAR protein expression in Leydig cells. Your thyroid hormone is literally controlling how much raw material gets into the factory where testosterone is built.

The third mechanism involves something called SHBG, which stands for sex hormone binding globulin, a protein your liver produces that binds to testosterone and makes it biologically unavailable. Your thyroid status directly affects how much SHBG your liver produces, which means that even when total testosterone looks adequate on paper, the amount of free testosterone your body can actually use is shaped by thyroid function. SHBG changes the context of every testosterone number on your bloodwork.

A conference abstract by Shrivastav and Saboo examined 51 men with overt hypothyroidism and found that 50 percent had low testosterone at baseline. After levothyroxine normalized their thyroid function, 70 percent of those men had their testosterone return to normal on its own. That is a conference abstract from a single center, not a peer reviewed trial, so it carries limited weight on its own, but it is directionally consistent with the Donnelly data and with the three mechanisms described above. Fix the thyroid, and the testosterone axis corrects itself.

The third axis, growth hormone, follows the same upstream dependence.

Your pituitary contains specialized cells called somatotrophs that produce growth hormone, and these cells need adequate T3 to express the receptors that receive the GHRH signal from your hypothalamus. Miki and colleagues showed that hypothyroidism depressed the growth hormone response to GHRH and that T3 treatment restored it. For people using growth hormone peptides like ipamorelin or MK-677, there is an additional layer. These peptides work by targeting something called the growth hormone secretagogue receptor on your pituitary cells. Kamegai and colleagues showed in 2001 that T3 increases the expression of this receptor by extending its mRNA half life from 8 hours to 15 hours, effectively doubling how long the receptor stays available. That was an in vitro study using rat pituitary cells, so it has not been confirmed in human trials, but the mechanism suggests that suboptimal thyroid function means fewer receptors for these peptides to activate.

So the order of operations looks like this: thyroid drives pituitary sensitivity, pituitary sensitivity drives LH output, LH output drives Leydig cell function, and Leydig cell function drives testosterone production. Growth hormone response runs through the same pituitary layer. A problem at the thyroid level propagates through every system downstream of it.

Before reaching for any hormonal intervention, a full thyroid panel should be part of the picture. TSH alone is not sufficient. You want TSH, free T3, free T4, and reverse T3 together, because that combination can reveal functional hypothyroidism that TSH alone will never show. Alongside that, total testosterone, free testosterone, LH, and SHBG give you the context to understand whether a low testosterone reading is coming from the testes themselves or from a broken relay upstream.

If suboptimal thyroid function is present, several nutritional factors are worth addressing before any medication. Selenium at 200 micrograms per day supports the deiodinase conversion of T4 to T3, with 400 micrograms as the upper limit not to exceed. Zinc at 30 milligrams daily is required for TRH synthesis and thyroid hormone receptor function. Iron is a structural component of thyroid peroxidase, the enzyme your thyroid uses to produce T4 and T3 in the first place, and research in rats showed that iron deficiency reduced that enzyme's activity by 33 to 56 percent, so ferritin is worth checking. Iodine is the building block of both T4 and T3 but excess iodine can worsen autoimmune thyroid conditions like Hashimoto's, so it should only be supplemented if deficiency has been confirmed.

Stress belongs in this conversation as well. Chronic cortisol activates the type 3 deiodinase enzyme and shifts T4 metabolism away from active T3 and toward reverse T3 within hours, as Chopra and colleagues documented in 1975. This is why someone can have normal TSH and normal T4 but still have the downstream hormonal profile of a hypothyroid person. The system looks intact from outside but the active signal is being diverted.

When natural optimization is not enough, it is not enough. Hashimoto's thyroiditis is autoimmune and does not respond to selenium and zinc. True hypothyroidism with elevated TSH and low T4 and T3 requires levothyroxine under medical supervision, and over replacement creates its own set of problems including bone loss, atrial fibrillation, and anxiety, which is why this requires a physician running a full panel rather than just managing TSH to a number.

Most men who discover they have low testosterone never get their thyroid checked. They get a testosterone number, it is low, and they get a prescription. The Leydig cells get bypassed entirely with exogenous testosterone, the feedback loop suppresses the body's own production, and the underlying problem, a thyroid axis that was quietly breaking the relay, goes untouched.

The foundation was broken the whole time. They just built on top of it anyway.

---

References

1. Donnelly P, White C. 2000. Testicular dysfunction in men with primary hypothyroidism; reversal of hypogonadotrophic hypogonadism with replacement thyroxine. Clinical Endocrinology, 522:197-201. Free testosterone nearly doubled 161 to 315 pmol/L after thyroxine replacement in hypothyroid men. Source
2. Shrivastav A, Saboo B. 2022. Effect of levothyroxine replacement therapy on testosterone, LH, FSH levels in men with overt hypothyroidism. ECE2022 Conference Abstract, Endocrine Abstracts, 81, P730. 70% of hypogonadal hypothyroid patients had testosterone normalize after levothyroxine. Conference abstract, N=51. Source
3. Maran RR, et al. 2000. Assessment of mechanisms of thyroid hormone action in mouse Leydig cells. Endocrinology, 14112:4468-4477. T3 increases LH receptor numbers, StAR protein 260% increase, and steroidogenic enzyme expression in Leydig cells. Source
4. Kamegai J, et al. 2001. Thyroid hormones regulate pituitary growth hormone secretagogue receptor gene expression. Journal of Neuroendocrinology, 133:275-278. T3 increased GHS-R mRNA by extending half-life from 8h to 15h in rat pituitary cells. Source
5. Miki N, et al. 1989. Effects of hypothyroidism, T3 and glucocorticoids on GH responses to GHRH. Journal of Endocrinology, 122:585-591. Hypothyroidism depressed growth hormone response to GHRH; T3 treatment restored it. Source
6. Winther KH, et al. 2020. Thyroid function in patients with selenium deficiency exhibits high free T4 to T3 ratio. BMC Endocrine Disorders. Selenium deficiency directly associated with impaired T4 to T3 conversion. Source
7. Hess SY, et al. 2002. Iron deficiency anemia reduces thyroid peroxidase activity in rats. Journal of Nutrition, 1327:1951-1955. Iron deficiency reduced TPO activity by 33-56%. Source
8. Chopra IJ, et al. 1975. Opposite effects of dexamethasone on serum concentrations of reverse T3 and T3. Journal of Clinical Endocrinology and Metabolism. Cortisol shifts T4 metabolism toward reverse T3 within hours. Source

---

Join the free community:
Men: Iron Forge Brotherhood
Women: Powerhouse Fitness