Thyroid Stimulating Hormone (TSH)

TSH is released by the pituitary to signal the thyroid to produce thyroxine (T4) and triiodothyronine (T3). Both underactive thyroid (hypothyroidism, high TSH) and overactive thyroid (hyperthyroidism, low TSH) can disrupt menstrual cycles and reduce fertility.

Most fertility specialists recommend a TSH below 2.5 mIU/L before starting IVF or during early pregnancy, as thyroid hormone is critical for fetal brain development. Subclinical hypothyroidism (TSH between 2.5–4.5 with normal T4) is treated with levothyroxine in women undergoing fertility treatment.

TSH is routinely checked in the infertility workup and should be monitored during pregnancy.