Thyroid dysfunction and testosterone abnormalities often coexist, and the relationship between them is bidirectional and sometimes overlooked. Men with hypothyroidism may have low testosterone that normalizes with thyroid treatment. Men with hyperthyroidism may have elevated SHBG that makes their free testosterone low despite normal total testosterone. Before attributing symptoms to primary testosterone deficiency, thyroid function should be part of the workup.

How Thyroid Hormones Affect Testosterone

Thyroid hormones (T3 and T4) affect testosterone through several mechanisms:

SHBG regulation: The liver produces SHBG, and thyroid hormone status directly influences hepatic SHBG production. Hyperthyroidism (excess thyroid hormone) raises SHBG, sometimes substantially. Elevated SHBG binds more testosterone, reducing the free fraction. A man with hyperthyroidism may have a normal total testosterone and low free testosterone with symptoms of androgen deficiency, even though his testosterone production is not impaired.

Hypothyroidism has the opposite effect: it reduces SHBG, which raises free testosterone relative to total. This means a hypothyroid man may have artificially low total testosterone with normal free testosterone, his symptoms (fatigue, low libido) come from thyroid deficiency, not androgen deficiency.

Direct testicular effects: Thyroid hormone receptors are present in Leydig cells. T3 and T4 are required for normal Leydig cell function. Severe hypothyroidism impairs testosterone synthesis directly. Animal models demonstrate that hypothyroidism reduces LH-stimulated testosterone production, an effect that reverses with thyroid hormone replacement.

HPG axis effects: Severe hypothyroidism can elevate prolactin (through elevated thyrotropin-releasing hormone, which also stimulates prolactin secretion). Elevated prolactin suppresses GnRH and thus testosterone. Treating hypothyroidism normalizes TRH, prolactin falls, and testosterone may recover.

When to Test Thyroid Function Alongside Testosterone

The Endocrine Society and most clinical endocrinology guidelines recommend testing TSH as part of the workup for sexual dysfunction and symptoms of androgen deficiency, because thyroid dysfunction commonly causes overlapping symptoms and may be the primary diagnosis.

The clinical scenario that most commonly leads to missed thyroid-testosterone interaction: a man with fatigue, reduced libido, and borderline-low testosterone is started on TRT without thyroid testing. If hypothyroidism was the underlying driver, TRT addresses the testosterone number but not the cause, and symptoms may persist or partially improve while the thyroid condition goes untreated.

Any man with new-onset sexual dysfunction or suspected low testosterone should have at minimum TSH measured alongside the testosterone panel. If TSH is abnormal, free T4 and sometimes free T3 are added for context.

Hypothyroidism and Male Fertility

Thyroid disorders affect male fertility through several pathways. Hypothyroidism has been associated with:

  • Reduced sperm motility and morphology
  • Elevated prolactin (through the TRH mechanism above) suppressing sperm production
  • Testicular atrophy in severe or prolonged hypothyroidism

For men with unexplained infertility, thyroid function testing is part of the standard male fertility workup.

Testosterone and Thyroid Interactions

The relationship also runs in the other direction: testosterone affects thyroid function.

High-dose exogenous testosterone (as in supraphysiologic use by bodybuilders) reduces SHBG, which also affects thyroid hormone binding. Thyroid hormones are partly bound to thyroid-binding globulin (TBG), and TBG production is influenced by sex steroid levels. The interaction is complex and generally produces modest effects on measured thyroid hormone levels in men on TRT.

More practically relevant: testosterone therapy raises hematocrit. In men with pre-existing cardiovascular risk, both testosterone and thyroid function affect cardiovascular parameters, and managing both conditions simultaneously requires attention to how interventions interact.

What to Do When Both Are Low

A man found to have both low testosterone and hypothyroidism faces a sequencing question: treat both simultaneously, or treat thyroid first and retest testosterone?

Most endocrinologists prefer treating hypothyroidism first and rechecking testosterone after 3-6 months of stable thyroid treatment. If testosterone normalizes with thyroid treatment, the testosterone deficiency was secondary to hypothyroidism. If testosterone remains low after thyroid function normalizes, primary hypogonadism may coexist and TRT becomes appropriate.

This sequencing avoids treating a condition that may resolve on its own, and it establishes the correct diagnosis before committing to long-term testosterone therapy.

For an overview of the complete hormone panel evaluation, see How to Read a Hormone Blood Panel: Key Values Explained. For the signs of low testosterone and when to seek evaluation, see 9 Signs of Low Testosterone That Men Frequently Dismiss.