You've had your testosterone tested. Total T came back 450 ng/dL. Your doctor said it's "in range." But you're tired, your libido is poor, you're gaining fat despite training. The problem might not be your testosterone at all — it might be your thyroid.
Here's why: the thyroid regulates metabolic rate and sex hormone binding globulin (SHBG). If your thyroid is underactive, your SHBG climbs. And when SHBG climbs, it binds testosterone and locks it away, leaving you with low free testosterone despite adequate total testosterone.
This is one of the most overlooked connections in men's health.
How the Thyroid Affects Testosterone
The thyroid makes two hormones: T3 and T4. These regulate your metabolic rate, mood, body composition, and — critically — they suppress SHBG production in the liver.
When thyroid function is low (hypothyroidism), SHBG production increases. SHBG is a binding protein that latches onto testosterone (and oestradiol and DHT). The more SHBG you have, the more testosterone gets bound and unavailable to your androgen receptors.
A practical example:
- Total testosterone: 500 ng/dL (normal)
- SHBG: 55 nmol/L (elevated due to hypothyroidism)
- Free testosterone: 6 ng/dL (LOW — symptomatic range)
Versus the same man with healthy thyroid function:
- Total testosterone: 500 ng/dL
- SHBG: 30 nmol/L (normal)
- Free testosterone: 11 ng/dL (adequate)
Same total T. Different outcomes. The thyroid is the difference.
This is why men with hypothyroidism often feel low-T symptoms despite adequate total testosterone.
Hypothyroidism and Metabolic Effects
Beyond SHBG, a sluggish thyroid also:
- Lowers your metabolic rate — you burn fewer calories, gain fat more easily, especially in the midsection
- Impairs testosterone synthesis — T3 is needed for proper Leydig cell function in the testis
- Increases oestradiol — low thyroid function impairs oestradiol metabolism, compounding the problem
- Tanks your mood and energy — depression and fatigue are cardinal symptoms
- Worsens cholesterol levels — elevated LDL, often resistant to statins
So a sluggish thyroid doesn't just affect testosterone production — it affects how well your existing testosterone works.
How Common Is Hypothyroidism in Men Over 40?
Subclinical hypothyroidism — TSH slightly elevated, but T3 and T4 still in "normal" range — affects approximately 4–6% of men over 40. Clinical hypothyroidism (TSH clearly high, T4 low) affects about 1–2%.
The kicker: the NHS often misses subclinical hypothyroidism. On the NHS, if your TSH is below 10 mIU/L, you're often told it's fine. But many endocrinologists and functional practitioners consider TSH >2.5 mIU/L as suboptimal, especially if you have symptoms.
What to Test
TSH (thyroid-stimulating hormone). Your pituitary makes TSH to tell your thyroid to produce more T3 and T4. If TSH is high, your thyroid is struggling.
- Reference range: typically 0.4–4.0 mIU/L
- Optimal range: 0.5–2.5 mIU/L
- Above 4.0: usually merits investigation and likely treatment
Free T4 (thyroxine). The active form your thyroid produces. Should be mid-to-high in the reference range.
- Reference range: typically 9–20 pmol/L (varies by lab)
- Optimal: 12–18 pmol/L
Free T3 (triiodothyronine). The most biologically active thyroid hormone. Often overlooked but critical.
- Reference range: typically 2.8–7.1 pmol/L
- Optimal: 4.5–6.5 pmol/L
TPO antibodies (thyroid peroxidase). If elevated, suggests autoimmune thyroid disease (Hashimoto's). Not always tested on the NHS, but useful information.
- Normal: <35 IU/mL
The NHS will typically only test TSH. If it's "normal" (by their standards), they'll stop there. You'll need private testing to get the full picture.
Common Causes of Hypothyroidism in Men
Iodine deficiency. Less common in the UK due to salt iodisation, but possible if you're avoiding salt or have poor diet.
Selenium deficiency. Selenium is needed to convert T4 to the active T3. Deficiency is subtle but impactful. Typical intake: 50–200 μg/day; Brazil nuts are an excellent source.
Autoimmune thyroid disease. Hashimoto's thyroiditis. Less common in men than women, but it happens.
Chronic stress and poor sleep. Prolonged stress impairs the hypothalamic-pituitary-thyroid axis.
Age. TSH tends to drift upward with age, and subclinical hypothyroidism becomes more common.
Medications. Some drugs (lithium, some anticonvulsants) impair thyroid function.
How to Get Tested
Private testing (recommended). Use Medichecks or similar. Order their "thyroid panel plus" or equivalent, which includes TSH, free T3, free T4, and TPO antibodies. Cost: £30–50. Takes 2–3 days.
NHS testing. Ask your GP for TSH. They'll usually only do TSH. If it's "normal," you may struggle to get further investigation. If it's clearly elevated (>4.0), your GP will typically refer to endocrinology or start treatment.
What to Do If Your Thyroid Is Low
If TSH is mildly elevated (2.5–4.0) with symptoms:
Try lifestyle first: ensure adequate iodine (sea vegetables, iodised salt), selenium (Brazil nuts, fish), sleep, stress management. Retest in 6–8 weeks.
If TSH is clearly elevated (>4.0) or you have confirmed low T3/T4:
Levothyroxine (synthetic T4) is the standard treatment. Start at 25–50 mcg/day and titrate up based on symptoms and TSH. Typical maintenance: 75–150 mcg/day for men. Full effect takes 6–8 weeks.
Some men — especially those with poor T4-to-T3 conversion — benefit from adding small amounts of T3 (Cytomel or liothyronine) or using desiccated thyroid extract (NDT), though this is more controversial. Discuss with your doctor.
Retest: After 6–8 weeks on levothyroxine, get TSH, free T3, and free T4 rechecked. Adjust dose until symptoms resolve and labs are optimal.
What Happens to Testosterone When You Fix Your Thyroid
Once thyroid function normalises:
- SHBG drops (usually by 20–30%)
- Free testosterone rises (often 30–50% improvement)
- Metabolic rate improves, making fat loss easier
- Mood, energy, and libido improve
- Oestradiol metabolism normalises
Many men who thought they had "low testosterone" find that treating their thyroid solves the problem entirely — without ever needing testosterone replacement.
The Bottom Line
If you have low testosterone symptoms but haven't had your thyroid tested, you're missing a critical piece of the puzzle. TSH alone isn't enough — get free T3 and free T4 too. Use private testing if your GP won't order it. Fixing your thyroid might be the single most impactful thing you do for your testosterone, energy, and body composition.
Test first. Treat second. Don't assume low testosterone until you've ruled out thyroid dysfunction.