How are TSH and fertility related?

TSH (thyroid-stimulating hormone) is a key regulator of thyroid function and influences fertility through multiple mechanisms. In reproductive medicine, a TSH level above 2.5 mIU/L is often considered elevated and is associated with prolonged time to pregnancy, ovulatory dysfunction, and increased miscarriage risk. Even subclinical hypothyroidism (TSH 2.5–4.0 mIU/L with normal peripheral hormones) can impair oocyte quality, implantation, and early pregnancy maintenance. In men, thyroid dysfunction negatively affects sperm quality and libido. The evidence is human-strong: numerous clinical studies and guidelines (e.g., American Thyroid Association, European Thyroid Association) recommend TSH screening in couples with infertility and levothyroxine therapy when TSH exceeds 2.5 mIU/L. Caveat: TSH alone is not an absolute fertility marker – individual factors such as age, ovarian reserve, cycle patterns, and partner factors must be considered. A normal TSH does not rule out other causes. Measurement should be done fasting in the morning due to diurnal variation. In summary, TSH and fertility are closely linked; optimizing thyroid function can improve fertility outcomes, but therapy should be individualized and medically supervised.