Can hyperthyroidism (characterized by low Thyroid-Stimulating Hormone (TSH)) increase Follicle-Stimulating Hormone (FSH) levels in men?

Hyperthyroidism and FSH Levels in Men

Yes, hyperthyroidism (characterized by low TSH) can raise FSH levels in men, particularly in cases of Graves' disease. 1 This hormonal relationship is part of the complex interplay between thyroid function and the hypothalamic-pituitary-gonadal axis.

Mechanism and Evidence

Hyperthyroidism affects the hypothalamic-pituitary-gonadal axis in several ways:

• Studies show that men with hyperthyroidism demonstrate increased gonadotropin secretion, with FSH specifically being elevated in hyperthyroid men 1
• This elevation occurs without changes to the pulsatile characteristics (frequency, peak shape) of FSH secretion 1
• Triiodothyronine (T3) directly modulates FSH synthesis and secretion in the pituitary, with thyrotoxicosis increasing Fshb mRNA content and its association with ribosomes 2

Clinical Implications

The elevation of FSH in hyperthyroid men has several clinical consequences:

• Reproductive function: Hyperthyroidism can negatively affect semen quality 3
• Hormonal balance: Men with hyperthyroidism typically show:
  • Elevated testosterone and SHBG concentrations 4
  • Elevated estradiol levels 4
  • Relative primary gonadal insufficiency due to exaggerated SHBG levels 1
  • Common development of gynecomastia 4

Diagnostic Considerations

When evaluating a man with suspected hyperthyroidism:

• Thyroid function tests should include TSH, Free T4, and Free T3 5
• Low TSH (<0.45 mIU/L) suggests hyperthyroidism, while very low TSH (<0.1 mIU/L) strongly indicates hyperthyroidism 5
• Consider measuring FSH, LH, testosterone, and SHBG to assess the impact on the reproductive axis
• Men with hyperthyroidism exhibit hyperresponsiveness of LH to GnRH administration and subnormal responses to hCG 4

Treatment Implications

Treating the underlying hyperthyroidism typically normalizes reproductive hormone abnormalities:

• Successful treatment of hyperthyroidism generally restores normal gonadotropin levels 6
• Beta-blockers can be used for symptomatic relief in patients with thyrotoxicosis 5
• Monitor thyroid function every 2-3 weeks during treatment 5

Caveats and Pitfalls

• Differential diagnosis: Other causes of low serum TSH with normal FT4 must be excluded, including:

  • Recovery phase after hyperthyroidism treatment 7
  • Nonthyroidal illness (euthyroid sick syndrome) 7
  • Medication effects (dopamine, glucocorticoids, dobutamine) 7
  • Pituitary or hypothalamic failure 7

• Monitoring: For patients with subclinical hyperthyroidism, repeat testing is recommended within:

  • 3 months for TSH 0.1-0.45 mIU/L
  • 4 weeks for TSH <0.1 mIU/L 5

The relationship between thyroid hormones and reproductive hormones underscores the importance of evaluating thyroid function in men presenting with reproductive or fertility concerns, as correcting thyroid dysfunction can often improve reproductive parameters.