Thyroid and Testosterone in Men: The Overlap Your Blood Test Can Miss

He is 46, tired by mid-afternoon, has put on six kilos without changing much, and his libido has faded. A private clinic tests his testosterone, finds it at 9 nmol/L, and offers him a prescription.

Nobody checks his thyroid. When his GP eventually does, his TSH is well above the normal range. Six months on levothyroxine later, his energy is back, the weight has started to shift, and his testosterone, retested, is 14.

That story is common enough to be worth an article of its own. An underactive thyroid looks remarkably like low testosterone, it can genuinely lower a testosterone result, and it is one of the cheapest things in medicine to test for.

Why the two get confused

Put the symptom lists side by side and the overlap is striking.

• Tiredness and low energy
• Weight gain, particularly when nothing obvious has changed
• Low mood and poor concentration
• Reduced interest in sex
• Muscle aches and weakness
• Dry skin and thinning hair

An underactive thyroid can also cause feeling cold, constipation, a hoarse voice and a slow pulse, which are less typical of low testosterone. But most men do not notice those, and few doctors ask.

Hypothyroidism is more common in women, which may be one reason it is less often suspected in men. It still affects plenty of them.

How the thyroid changes a testosterone result

This is the part most men, and some clinicians, do not know.

The SHBG effect

Most testosterone in your blood is bound to a carrier protein called SHBG. A standard testosterone test measures the total: bound and unbound together.

Thyroid hormone affects how much SHBG the liver makes. An underactive thyroid lowers SHBG, so there is less protein for testosterone to bind to, and the total testosterone result falls. Low free testosterone has also been found in men with hypothyroidism, and it tends to return to normal once the thyroid is treated.

An overactive thyroid does the opposite. SHBG rises and total testosterone often looks normal or even high, while the free testosterone actually available to tissues is usually normal. An overactive thyroid also speeds up the conversion of testosterone to oestrogen, which can cause breast tissue to grow in some men.

Either way, the total testosterone number on its own can mislead you. Our guides to SHBG and free testosterone explain why the bound and free figures matter.

The prolactin effect

When the thyroid is underactive, the brain releases more of a signalling hormone to push it harder. That same signal also stimulates prolactin. In a minority of men, hypothyroidism raises prolactin enough to suppress testosterone, and treating the thyroid brings both back down. We cover prolactin in detail in prolactin: the test that changes the diagnosis.

Erections and ejaculation

A multicentre Italian study by Carani and colleagues, published in 2005, looked at sexual symptoms in men with thyroid disease. Erectile problems were common in both underactive and overactive thyroid. The two conditions also tended to affect ejaculation in opposite ways: delayed ejaculation was more common with an underactive thyroid, and premature ejaculation with an overactive one. Importantly, many of these problems improved once thyroid function was corrected.

If erections have become unreliable, the thyroid belongs on the list of things to check, alongside the more familiar causes in low testosterone and erectile dysfunction.

Getting the right tests

What to ask for

The standard first test is TSH, thyroid stimulating hormone. A high TSH usually means an underactive thyroid; a low TSH usually means an overactive one.

But TSH alone can mislead in one important situation. If the problem lies in the pituitary gland rather than the thyroid itself, TSH can be normal or low even though the thyroid is underactive. That is called central hypothyroidism, and it matters here because the same pituitary problems can also cause low testosterone. Reviewers in Translational Andrology and Urology described missing it as one of the commonest errors they see.

So if you are being investigated for low testosterone, especially with a low or normal LH, ask for free T4 alongside TSH, not TSH alone.

One thing that can spoil the result

High-dose biotin, common in hair, skin and nail supplements, can interfere with many laboratory immunoassays, including some thyroid tests and some hormone tests. The result can look abnormal when it is not. If you take biotin, tell whoever orders the test, and ask whether to stop it for a few days beforehand.

Some herbal supplements can also affect thyroid levels. Ashwagandha, often taken for testosterone, has been reported to raise thyroid hormone levels in small studies, which is worth knowing if you have a thyroid condition or are about to be tested.

The borderline result

Many men get a result that sits in between: a TSH slightly above the normal range but a normal free T4. This is called subclinical hypothyroidism. It is common, it sometimes returns to normal on its own, and it does not always need treatment.

NICE advises repeating the test, usually after about three months, before deciding. Whether to treat depends on how high the TSH is, your age and your symptoms. If the TSH is only mildly raised, a doctor may suggest a trial of treatment to see whether symptoms improve, or simply monitoring. It is a conversation worth having rather than a box to tick.

Testing for thyroid antibodies can help. Their presence suggests an autoimmune thyroid condition, which is more likely to progress over time.

Which to treat first

If you have both a thyroid problem and low testosterone, the usual order is:

1. Treat the thyroid first. Levothyroxine for an underactive thyroid is cheap, well tested and usually taken as a single daily tablet.
2. Wait until thyroid levels have been stable for a few months.
3. Repeat the testosterone test, in the morning, with SHBG.
4. Only then decide whether testosterone needs treatment in its own right.

Starting testosterone before the thyroid is sorted risks treating a number that would have corrected itself, and adds a long-term treatment you might not need. It can also hide improvement, making it impossible to tell which treatment helped.

If testosterone is still low once the thyroid is normal, the investigation continues as usual. Our guide to what causes low testosterone lists the other causes worth excluding.

Why this is worth pushing for

Thyroid tests are routine, inexpensive and widely available. If you have symptoms that sound like low testosterone, and your thyroid has not been checked recently, it is a reasonable thing to ask for. If you are already being offered testosterone and have not had a thyroid test, ask why.

For the broader picture of which blood tests to request, see the testosterone blood test and my doctor said normal but I feel tired.

Common questions

Can an underactive thyroid cause low testosterone?

Yes. It lowers SHBG, which reduces total testosterone, and has been linked to low free testosterone that returns to normal once the thyroid is treated. In some men it also raises prolactin, which suppresses testosterone.

Can an overactive thyroid affect testosterone?

Yes. It raises SHBG, so total testosterone may look normal or high while free testosterone is usually normal. It can also increase conversion to oestrogen and cause breast tissue growth.

What thyroid tests should a man with low testosterone have?

TSH and free T4 together. TSH alone can miss thyroid problems caused by the pituitary gland, which can also lower testosterone.

Should I start testosterone if my thyroid is also abnormal?

Usually not yet. Treat the thyroid, wait for levels to settle, then retest testosterone before deciding.

Can supplements affect my thyroid or hormone results?

Yes. High-dose biotin can distort many lab tests, and some herbal supplements, including ashwagandha, have been reported to raise thyroid hormone levels.

Sources: Crawford M, Kennedy L, Testosterone replacement therapy: role of pituitary and thyroid in diagnosis and treatment, Transl Androl Urol 2016;5(6) · Meikle AW, The interrelationships between thyroid dysfunction and hypogonadism in men and boys, Thyroid 2004;14(Suppl 1) · National Institute for Health and Care Excellence, Thyroid disease: assessment and management (NG145) · Carani C et al., Multicenter study on the prevalence of sexual symptoms in male hypo- and hyperthyroid patients, J Clin Endocrinol Metab 2005;90(12):6472-9 · Educational only, not medical advice.

Keep reading: What causes low testosterone · Prolactin: the test that changes the diagnosis · What is SHBG? · Doctor said normal but you feel tired · Blood test: what to check · Low testosterone and depression