What role does TSH play in fertility?

TSH (thyroid-stimulating hormone) plays a key role in fertility by regulating thyroid function. Both hypothyroidism (high TSH) and hyperthyroidism (low TSH) can disrupt menstrual cycles, ovulation, and sperm quality. Evidence from large observational studies and clinical guidelines indicates that even subclinical TSH elevations (>2.5 mIU/L) are associated with reduced fertility. For women trying to conceive, a target TSH <2.5 mIU/L is commonly recommended. However, TSH alone is not a direct fertility marker; free T3, free T4, and thyroid antibodies (TPO-Ab) are also important. TSH testing is a useful part of fertility workup but insufficient for a complete assessment. Caveat: A normal TSH does not rule out thyroid dysfunction, and individual responses vary. The evidence is strong (human-strong) but based on observational data, not randomized trials specifically targeting TSH optimization for fertility.