Your TRT Clinic Didn't Check the One Hormone That Matters Most

Testosterone shows up low on a lab. A clinic hands you a prescription. And nobody asks why it was low in the first place.

That is the problem this article is about.

To understand it, you need to start with the full system before zooming into any single part of it. Your body runs testosterone production through a chain that starts in the brain and ends in your testes. The hypothalamus sends a signal called GnRH down to the pituitary gland, which then releases something called LH, which is luteinizing hormone, the chemical messenger that travels through the bloodstream and tells the Leydig cells in your testes to produce testosterone. That whole chain is called the HPG axis, which is the hypothalamic-pituitary-gonadal axis, and it is the system your testosterone depends on at every step.

Now here is where thyroid enters the picture.

Your thyroid produces two hormones. The first is T4, which is the storage form, a mostly inactive molecule your body holds in reserve. The second is T3, which is the active form, the version your cells can actually use. T4 has to be converted into T3 before your body can do anything with it, and that conversion happens through enzymes called deiodinases, which strip an iodine atom off the T4 molecule to activate it.

T3 does not just run your metabolism. It sits at three separate points along your testosterone production chain, and when it is low, all three points fail at once.

The first point is the pituitary. T3 directly affects how well the pituitary responds to the GnRH signal coming from the hypothalamus. When thyroid function is low, the pituitary receives the signal but does not pass it along with the same force. LH output drops. And when LH drops, the testes get a weaker instruction to produce testosterone, so they produce less.

The second point is inside the Leydig cell itself. This is where the detail gets specific. Inside the Leydig cell, testosterone synthesis begins when cholesterol gets transported into the mitochondria. That transport step is controlled by something called StAR protein, which stands for steroidogenic acute regulatory protein, and it is the rate-limiting step in the whole process, meaning nothing downstream can go faster than this one step allows. T3 directly controls how much StAR protein your Leydig cells express. Research in mouse Leydig cells found that T3 drives a 260 percent increase in StAR protein expression, along with an increase in the number of LH receptors on the cell surface, which means not only can the cell make more testosterone when T3 is present, it can also hear the LH signal more clearly.

Think of it like a factory. The workers are there, the raw materials are stacked at the door, but the power is out. Nothing moves. That is what low T3 does to a Leydig cell.

The third point is in the liver. Your liver produces something called SHBG, which is sex hormone-binding globulin, a protein that binds to testosterone in your bloodstream and makes it unavailable to your tissues. Bound testosterone cannot act on your cells. Only free testosterone can. Thyroid hormones regulate how much SHBG your liver produces, and when thyroid function is low, SHBG tends to rise, which means even the testosterone you do produce gets captured before it can do anything.

So you have three mechanisms running simultaneously: weaker LH signal from the pituitary, slower production inside the Leydig cell, and more of what you do produce getting bound and neutralized.

What does this look like in practice? A study published in Clinical Endocrinology followed men with primary hypothyroidism and measured their testosterone before and after treating their thyroid with thyroxine replacement. Free testosterone nearly doubled, moving from 161 to 315 pmol/L, with no testosterone therapy involved at any point. The only intervention was fixing the thyroid.

That number matters because it shows the relationship is not subtle. These were not men at the margins. Their testosterone was genuinely suppressed by hypothyroidism, and it recovered when the underlying cause was addressed.

Now here is the part that makes this a practical problem. Standard thyroid screening usually only measures TSH, which is thyroid-stimulating hormone, the signal the pituitary sends to the thyroid to produce more hormone. TSH alone can look normal while your actual T3 levels are low, particularly if the problem is not production but conversion. Your thyroid might be making enough T4, but if the deiodinase enzymes that convert it to T3 are impaired, your TSH stays normal while your active hormone is deficient.

This is why a full panel matters. You want TSH, free T4, free T3, and reverse T3, which is an inactive form of thyroid hormone that competes with T3 at the receptor level and can block its effects even when T3 itself looks adequate. If any of these are off, you are not looking at the complete picture with TSH alone.

One specific reason conversion can fail even when the thyroid itself is functioning is selenium deficiency. The deiodinase enzymes that convert T4 to T3 require selenium to work, and research has found that selenium deficiency is directly associated with an elevated free T4 to free T3 ratio, meaning the raw material is there but the conversion is not happening. Selenium is not a supplement most people think about in the context of testosterone, but deficiency creates exactly the pattern where TSH looks fine, T4 looks fine, and free T3 is quietly suppressed.

The practical sequence is straightforward. If testosterone is low, check thyroid first and check it completely. If free T3 is low, if conversion is impaired, if SHBG is elevated and thyroid function is a likely driver, address that before adding testosterone to a system that has not been diagnosed correctly.

Testosterone therapy on top of unaddressed hypothyroidism does not fix the underlying mechanism. The Leydig cells are still running without enough power. The SHBG problem is still there. The pituitary signal is still blunted. You are supplementing around a problem that could have been solved at the source.

The question was never just "is testosterone low?" The question is always "why is it low?" and a testosterone reading without a thyroid panel is only half an answer.

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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. Source
2. 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 and StAR protein expression 260% increase in Leydig cells. Source
3. Krassas GE, et al. 2010. The male and female reproductive systems in hypothyroidism. Thyroid hormones modulate HPG axis at multiple levels including pituitary LH response, direct Leydig cell effects, and SHBG regulation. Source
4. 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

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