How does hyperthyroidism affect Follicle-Stimulating Hormone (FSH) levels in men?

Hyperthyroidism and FSH Levels in Men

In men with hyperthyroidism, FSH levels are typically normal or may show no significant elevation, though the gonadotropin response to GnRH stimulation is enhanced; these changes normalize with treatment of the hyperthyroid state. 1, 2

FSH Levels in Hyperthyroid Men

Baseline FSH Concentrations

• FSH levels remain within normal range in most hyperthyroid men, showing no significant difference compared to euthyroid controls at baseline. 2
• Unlike the marked elevations seen in primary hypothyroidism (where FSH is elevated and associated with testicular enlargement), hyperthyroidism does not produce consistent FSH elevation. 1

Enhanced Gonadotropin Responsiveness

• Men with hyperthyroidism exhibit hyperresponsiveness of LH (and by extension FSH) to GnRH administration, contrasting with the subnormal responses seen in hypothyroid men. 1
• This enhanced pituitary sensitivity occurs despite normal baseline gonadotropin levels. 1

Mechanism: T4 vs T3 Effects

• Thyroxine (T4) appears more important than triiodothyronine (T3) for gonadotropin regulation. 3
• When normal women were given T4 supplementation, basal LH and FSH responses to GnRH increased significantly, mimicking the pattern seen in spontaneous hyperthyroidism. 3
• In contrast, T3 administration (even at doses producing serum T3 levels similar to hyperthyroid patients) did not alter basal or GnRH-stimulated LH and FSH levels. 3

Associated Hormonal Changes

Testosterone and Estradiol

• Serum testosterone (T) and estradiol (E2) levels are significantly elevated in hyperthyroid men (P < 0.005 compared to controls). 2
• These elevations reflect adaptation to the hypermetabolic state and changes in sex hormone-binding globulin (SHBG), which is increased in hyperthyroidism. 1, 2
• Gynecomastia is common in hyperthyroid men due to elevated estradiol concentrations. 1

Luteinizing Hormone

• LH levels show no significant difference at baseline between hyperthyroid men and controls, similar to FSH. 2
• However, LH demonstrates subnormal responses to hCG stimulation in hyperthyroid men, contrasting with the hyperresponsiveness to GnRH. 1

Reversibility with Treatment

Normalization of Hormone Levels

• All sexual hormone abnormalities, including any FSH alterations, return to normal after successful treatment of hyperthyroidism. 2
• There is no significant difference in FSH, LH, testosterone, or estradiol levels between treated patients and controls (P > 0.05). 2
• The hormonal changes represent functional adaptations to the hyperthyroid state rather than permanent gonadal axis damage. 1, 2

Clinical Pitfalls

Avoid Misdiagnosis

• Do not confuse the gonadotropin patterns of hyperthyroidism with primary hypogonadism—baseline FSH is normal in hyperthyroidism, not elevated. 1, 2
• The enhanced GnRH responsiveness in hyperthyroidism is opposite to the subnormal response seen in primary hypothyroidism. 1

Radioactive Iodine Considerations

• RAI therapy for hyperthyroidism can produce dose-dependent impairment of spermatogenesis with FSH elevation lasting up to 2 years. 1
• High-dose RAI may cause permanent testicular germ cell damage, though Leydig cell function typically recovers by 18 months post-therapy. 1
• This represents a treatment complication rather than a direct effect of hyperthyroidism itself. 1