Can thyroid dysfunction increase Follicle-Stimulating Hormone (FSH) levels in men?

Can Thyroid Dysfunction Increase FSH in Men?

Yes, thyroid dysfunction can increase FSH levels in men, but the effect differs dramatically based on the type of thyroid disorder: primary hypothyroidism typically elevates FSH (particularly in children and adolescents), while hyperthyroidism increases FSH in adult men as part of a broader disruption of the hypothalamic-pituitary-gonadal axis.

Primary Hypothyroidism and FSH

In Children and Adolescents

• Primary hypothyroidism in male children causes elevated FSH levels associated with testicular enlargement without virilization 1
• This represents a unique presentation where FSH elevation occurs despite the absence of secondary sexual characteristics 1

In Adult Men

• Adult men with primary hypothyroidism develop hypogonadotropic hypogonadism, which is characterized by subnormal LH responses to GnRH stimulation 1
• FSH levels in adult men with primary hypothyroidism are typically not as dramatically elevated as in children 2
• During the hypothyroid phase, men show hypergonadotropism with mean FSH levels of 6.3 ± 2.0 IU/L, which normalize to 2.7 ± 0.9 IU/L after thyroid hormone replacement 2
• The reproductive dysfunction is completely reversible with thyroid hormone replacement therapy 1, 2

Hyperthyroidism and FSH

Mechanism of FSH Elevation

• Men with hyperthyroidism exhibit increased FSH secretion as part of a broader disruption of gonadotropin regulation 3
• The hypothalamic-pituitary-gonadal axis remains functional but operates at elevated levels 3
• Hyperthyroid men demonstrate hyperresponsiveness of LH to GnRH administration, with FSH secretion also increased 1, 4

Clinical Manifestations

• Basal plasma FSH levels are elevated in hyperthyroid men, though the FSH response to LRH (GnRH) remains within normal range 4
• The pulsatile characteristics of FSH secretion (frequency and peak shape) are preserved despite elevated baseline levels 3
• Significant copulsatility occurs between LH and FSH, which is more pronounced in hyperthyroid patients than in healthy controls 3

Impact on Testicular Function

• Hyperthyroid men show evidence of impaired spermatogenesis, with total sperm counts less than 40 million in the majority of affected men 4
• There is an inverse correlation between serum estradiol levels and total sperm count (r = -0.87; P < 0.05) 4
• Despite elevated FSH, hyperthyroid men demonstrate partial Leydig cell failure with blunted testosterone response to hCG stimulation (80% increase vs. 193% in controls) 4

Radioactive Iodine Therapy Effects

• RAI therapy for thyroid cancer produces dose-dependent impairment of spermatogenesis with FSH elevation lasting up to approximately 2 years 1
• RAI commonly increases FSH and LH concentrations while reducing inhibin B levels without affecting testosterone 1
• Permanent testicular germ cell damage may occur with high-dose RAI, though most men recover germinal and Leydig cell function by 18 months post-therapy 1

Clinical Interpretation

Key Diagnostic Considerations

• FSH elevation in the context of thyroid dysfunction represents a secondary effect on the hypothalamic-pituitary-gonadal axis rather than primary testicular failure 1, 3
• The normal FSH range is generally 1.5-8 mIU/L in men 5
• Serial measurements are essential, as TSH and gonadotropin levels can vary by up to 50% on a day-to-day basis 5

Reversibility

• The FSH elevation associated with primary hypothyroidism is completely reversible with thyroid hormone replacement therapy 1, 2
• Improvement in sperm count and motility accompanies normalization of FSH levels after achieving euthyroid status 2
• For hyperthyroidism, treatment of the underlying thyroid disorder typically normalizes the hypothalamic-pituitary-gonadal axis function 1

Important Caveats

• Do not confuse thyroid dysfunction-related FSH elevation with primary testicular failure—the mechanisms and prognoses are entirely different 1
• Prolactin is not consistently elevated in primary hypothyroidism except in cases of longstanding severe disease 1
• Free testosterone concentrations are reduced in primary hypothyroidism despite variable FSH changes, and normalize with thyroid hormone replacement 1
• In hyperthyroidism, elevated SHBG levels create relative primary gonadal insufficiency despite preserved testicular function 3