Can Hyperthyroidism Cause Elevated FSH Levels?
Yes, hyperthyroidism can cause elevated FSH levels, as demonstrated in clinical studies where hyperthyroid women showed increased basal plasma LH concentrations and enhanced LH and FSH responses to GnRH compared to normal women. 1
Mechanism and Evidence
Thyroid hormones significantly influence the hypothalamic-pituitary-gonadal axis. The relationship between hyperthyroidism and elevated FSH is supported by several key findings:
In hyperthyroid women studied during the follicular phase of the menstrual cycle, both basal and GnRH-stimulated FSH levels were increased compared to normal women 1
These elevated gonadotropin levels normalized during or after treatment of hyperthyroidism, confirming the causal relationship 1
Interestingly, experimental administration of T4 (thyroxine) to normal women reproduced the elevated gonadotropin pattern seen in hyperthyroidism, while T3 (triiodothyronine) administration did not, suggesting that serum T4 specifically plays a greater role in gonadotropin regulation 1
Clinical Implications
The impact of hyperthyroidism on reproductive hormones has several important clinical implications:
Men with hyperthyroidism exhibit:
- Elevated concentrations of testosterone and sex hormone-binding globulin (SHBG)
- Elevated estradiol levels
- Hyperresponsiveness of LH to GnRH administration
- Subnormal responses to hCG
- Common occurrence of gynecomastia 2
In women, hyperthyroidism can lead to:
- Alterations in reproductive organ cytoarchitecture
- Reduced serum progesterone levels
- Decreased serum luteinizing hormone and estradiol 3
Differential Diagnosis of Elevated FSH
When interpreting elevated FSH levels in patients with regular menstrual cycles, it's important to consider hyperthyroidism among other potential causes:
- Physiological causes: puberty, hereditary dizygotic twinning, post-oral contraceptive use, lactation
- Pathological causes: unilateral ovariectomy, recovery from hypothalamic amenorrhea, excessive smoking 4
Management Considerations
When managing patients with concurrent hyperthyroidism and elevated FSH:
Treat the underlying hyperthyroidism, which will typically normalize FSH levels 1
Monitor for potential cardiac complications, as hyperthyroidism:
- Decreases systemic vascular resistance
- Increases resting heart rate and LV contractility
- Can lead to pulmonary artery hypertension 5
Be aware that treatment of hyperthyroidism with radioactive iodine can transiently impair both germinal and Leydig cell function in men, with recovery usually occurring by 18 months post-therapy 2
Caution
In rare cases, hyperthyroidism has been associated with sexual precocity, as reported in a case of neonatal hyperthyroidism that later developed breast development, bone age advancement, and elevated FSH and estradiol levels 6. This highlights the complex interplay between thyroid dysfunction and the reproductive axis.