Thyroid hormone and testosterone regulate each other in both directions. Untreated hypothyroidism can lower testosterone and produce symptoms, such as fatigue, weight gain, and low libido, that look identical to low T. Starting testosterone therapy can also change how much thyroid medication a patient needs, because testosterone lowers the blood protein that carries thyroid hormone. Neither condition should be diagnosed or treated in isolation.
How Does Thyroid Function Affect Testosterone Levels?
Hypothyroidism is associated with low total testosterone, largely because low thyroid function reduces sex hormone-binding globulin (SHBG), the protein that carries most testosterone through the bloodstream. Some men with hypothyroidism also have genuinely low free testosterone that normalizes once thyroid hormone levels are corrected (Crawford, Translational Andrology and Urology, 2016).
Hyperthyroidism works in the opposite direction. Excess thyroid hormone raises SHBG and total testosterone, but free testosterone usually stays in the normal range. Hyperthyroidism can also increase the conversion of testosterone into estrogen, which is one reason some men with overactive thyroids develop gynecomastia or sexual side effects despite normal-looking testosterone numbers on paper (Crawford, Translational Andrology and Urology, 2016).
A 2022 study presented at the European Congress of Endocrinology followed 51 men with overt hypothyroidism. Half had low testosterone at baseline. After four months of levothyroxine treatment restored normal thyroid function, 70% of the men who started out hypogonadal had their testosterone levels return to normal, with no testosterone therapy involved at all (Shrivastav & Saboo, Endocrine Abstracts, 2022).
Can Low Thyroid Function Cause Symptoms That Look Like Low T?
Yes, and this is the overlap that trips up a lot of self-diagnosis. Fatigue, weight gain, depressed mood, brain fog, low libido, and erectile dysfunction show up on the symptom list for both hypothyroidism and low testosterone. A man who feels flat, gains weight easily, and has lost interest in sex could be dealing with either condition, or both at once.
The overlap is not rare. One study of 120 men evaluated for erectile dysfunction found undiagnosed hypothyroidism in 5% of them and undiagnosed hyperthyroidism in another 1% (Crawford, Translational Andrology and Urology, 2016). That is a meaningful share of men whose "low T symptoms" were actually a thyroid problem the whole time. This is exactly why a thorough TRT workup checks more than a single testosterone number.
Does Starting TRT Change How Much Thyroid Medication You Need?
Often, yes, if you are already on levothyroxine or another thyroid replacement. Testosterone therapy lowers thyroxine-binding globulin (TBG), the protein that carries thyroid hormone in the blood. With less TBG in circulation, men on stable thyroid medication can end up needing a smaller dose once TRT is underway. Guidance from the Pituitary Foundation recommends rechecking thyroid labs 6 to 8 weeks after starting testosterone therapy, since the levothyroxine dose sometimes needs to come down by 25% to 50% to avoid symptoms of excess thyroid hormone (The Pituitary Foundation, UK).
In practice, this means telling your prescriber about any thyroid medication before you start TRT, and planning a follow-up thyroid panel a month or two after your first testosterone dose, not just a testosterone recheck.
Should You Get Your Thyroid Tested Before Starting TRT?
It is worth asking for one. A reputable TRT workup should already include a TSH along with the standard panel of total and free testosterone, LH, FSH, prolactin, estradiol, hematocrit, and PSA, precisely because thyroid dysfunction is a common, reversible cause of a low testosterone reading. Some men who assume they need testosterone therapy actually need levothyroxine instead, and their testosterone corrects on its own once thyroid hormone is back in range.
If your workup only measured testosterone and skipped thyroid function, ask your provider to add a TSH and free T4 before committing to a treatment plan. It is a single extra blood draw that can change the entire diagnosis.
