Short answer: It can change some of them. Testosterone labels say androgens may lower thyroxine-binding globulin (TBG), the protein that carries thyroid hormone, so total T4 can read lower while free thyroid hormone stays the same and there is no sign of thyroid disease. In people already on levothyroxine, a small 1994 study found androgen therapy raised free T4 and doses had to be cut by 25% to 50%.
The Depo-Testosterone label, under lab test interference, says androgens may decrease levels of thyroxine-binding globulin, resulting in lower total T4 and increased resin uptake of T3 and T4. It adds that free thyroid hormone levels remain unchanged and there is no clinical evidence of thyroid problems. The AndroGel label says the same.
The Synthroid (levothyroxine) label lists androgens and anabolic steroids among drugs that lower TBG. For someone taking levothyroxine, it says these drugs cause a brief early rise in free T4, and with continued use, total T4 falls while free T4 and TSH stay normal.
| Test | What it measures | Expected change with androgens |
|---|---|---|
| Total T4 | All T4 in the blood, bound and free | Can fall, because there is less carrier protein |
| T3 resin uptake | An older test of binding capacity | Can rise |
| Free T4 | The active, unbound hormone | Unchanged in people with a normal thyroid |
| TSH | The pituitary signal that controls the thyroid | Unchanged in people with a normal thyroid |
A 2009 review by Tahboub and Arafah explains that sex hormones change thyroid tests mainly by changing TBG: estrogen raises it and androgens lower it. People with a normal thyroid keep normal free T4 and TSH. The review says people with thyroid disease can need dose changes, and suggests checking thyroid function 6 to 8 weeks after starting or stopping sex hormone treatment.
The clearest data come from a small 1994 study by Arafah of women with breast cancer treated with an oral androgen, fluoxymesterone. In 7 women without thyroid disease, total T4 and TBG fell within 4 weeks while free hormone stayed the same, and the levels returned to baseline 6 to 12 weeks after stopping. In 4 women on long-term thyroid replacement, free T4 rose and TSH fell within 4 weeks, they developed signs of an overactive thyroid, and their thyroid doses had to be reduced by 25% to 50%.
This study was 11 women on a different androgen, not men on testosterone therapy, so it shows the direction of the effect rather than the size to expect on TRT. For someone on both medicines, the timing of thyroid tests after a testosterone start, stop or dose change is up to the prescriber. Estrogen does the opposite; see estrogen and thyroid medicine.
A low total T4 on its own, with normal free T4 and TSH, can reflect lower binding protein rather than a thyroid problem. That is easier to judge when the lab knows about every hormone being taken. Dose Track can keep testosterone injection dates and lab dates on the same timeline.
Read more on Dose Track: Free T4 · TSH · Hypothyroidism · Depo-Testosterone side effects · Low testosterone