Does iatrogenic hyperthyroidism increase Follicle-Stimulating Hormone (FSH) levels?

Effect of Iatrogenic Hyperthyroidism on FSH Levels

Iatrogenic hyperthyroidism does not increase Follicle-Stimulating Hormone (FSH) levels; rather, it tends to decrease FSH levels in both males and females.

Relationship Between Thyroid Function and Reproductive Hormones

• Thyroid dysfunction significantly impacts the hypothalamic-pituitary-gonadal axis, affecting reproductive hormone levels including FSH 1
• Hyperthyroidism (including iatrogenic) leads to decreased FSH levels in both males and females 2
• Animal studies have demonstrated that experimentally induced hyperthyroidism results in reduced serum FSH levels compared to euthyroid controls 1, 2

Mechanisms of FSH Suppression in Hyperthyroidism

• Excess thyroid hormones alter the hypothalamic-pituitary feedback mechanisms that regulate gonadotropin release 1
• In hyperthyroidism, there is hyperresponsiveness of Luteinizing Hormone (LH) to Gonadotropin-Releasing Hormone (GnRH) administration but subnormal responses to human Chorionic Gonadotropin (hCG) 3
• These altered feedback mechanisms contribute to the suppression of FSH secretion 2

Sex-Specific Effects of Hyperthyroidism on Reproductive Hormones

In Males:

• Hyperthyroidism leads to elevated concentrations of testosterone and Sex Hormone-Binding Globulin (SHBG) 3, 4
• Despite increased testosterone, FSH levels are reduced due to altered feedback mechanisms 2
• Estradiol elevations are commonly observed in men with hyperthyroidism, and gynecomastia is a frequent finding 3

In Females:

• Hyperthyroidism causes reduced estradiol and progesterone levels 1
• FSH levels are typically decreased in hyperthyroid states compared to euthyroid controls 1, 2
• The reduction in FSH contributes to menstrual irregularities and potential fertility issues in hyperthyroid women 1

Clinical Implications

• Treatment of hyperthyroidism with antithyroid medications (such as methimazole) or definitive therapy (radioactive iodine or surgery) can normalize FSH levels 5
• For symptomatic iatrogenic hyperthyroidism (grade 2), guidelines recommend interrupting the causative therapy and starting beta-blocker therapy 5
• Oral prednisolone (0.5-1 mg/kg) may be required short-term for thyroid gland inflammation or if symptoms are severe 5
• Monitoring reproductive hormone levels, including FSH, may be warranted in patients with iatrogenic hyperthyroidism, especially if fertility concerns exist 1

Reversal of Hormonal Changes with Treatment

• Normalization of thyroid function through appropriate treatment can restore normal FSH levels 2
• In experimental models, administration of replacement doses of thyroid hormone to hypothyroid animals restored FSH to normal levels, suggesting that correcting thyroid dysfunction normalizes gonadotropin secretion 2
• Clinical studies have shown that treating hyperthyroidism leads to normalization of SHBG and other hormone-binding proteins, which indirectly affects free hormone levels 4

Important Considerations and Pitfalls

• The relationship between thyroid hormones and reproductive hormones is complex and bidirectional 5
• Laboratory assessment of reproductive hormones in patients with thyroid dysfunction may be confounded by alterations in binding proteins 4
• SHBG levels are increased in hyperthyroidism, which affects the interpretation of total hormone levels versus free (bioavailable) hormone levels 4
• When evaluating reproductive function in patients with iatrogenic hyperthyroidism, both FSH and other reproductive hormones should be assessed for a complete picture 1, 3