The thyroid gland and the reproductive system are intimately connected, yet this relationship is often overlooked in clinical practice. Thyroid disorders — particularly hypothyroidism — are significantly more common in women than men, and they frequently manifest first as changes in the menstrual cycle.
Thyroid hormones influence nearly every cell in the body, including the cells of the ovaries, uterus, and pituitary gland. Thyroid-stimulating hormone (TSH) is structurally similar to follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Elevated TSH can stimulate prolactin production, which in turn suppresses the LH surge required for ovulation. This is one mechanism through which hypothyroidism causes irregular or absent periods.
An underactive thyroid can manifest in the menstrual cycle as heavy periods, irregular cycles, frequent periods, or absent periods. The classic presentation of untreated hypothyroidism includes fatigue, weight gain, cold intolerance, hair loss, dry skin, constipation, and brain fog — symptoms that overlap substantially with other common women's health conditions and are frequently attributed to stress or aging.
The prevalence of hypothyroidism in women is approximately 5-10 times higher than in men, with the greatest risk in women over 35 and in the postpartum period. Postpartum thyroiditis affects approximately 5-10% of women in the year after birth and can present as a phase of hyperthyroidism followed by hypothyroidism, or as hypothyroidism alone.
Hashimoto's thyroiditis is the most common cause of hypothyroidism in developed countries. Women with Hashimoto's have elevated thyroid peroxidase (TPO) antibodies. Research has shown that elevated TPO antibodies are associated with increased risk of miscarriage, even in women whose thyroid hormone levels are technically within normal range. This has led some fertility specialists to recommend TSH targets below 2.5 mIU/L for women trying to conceive, more stringent than the standard laboratory normal range.
An overactive thyroid can cause lighter, shorter, or absent periods. Other symptoms include heart palpitations, weight loss despite good appetite, heat intolerance, anxiety, and fine tremor. Graves' disease is the most common cause and also disproportionately affects women.
Thyroid screening should be considered for any woman experiencing: irregular periods without another explanation, unexplained weight changes, persistent fatigue and cold intolerance, hair loss, recurrent miscarriage, difficulty conceiving, or a family history of thyroid disease. A basic thyroid panel includes TSH as the primary screen. Many women have spent years managing the symptoms of undiagnosed thyroid disease. A TSH test can be diagnostic. If you identify with the symptoms described here, advocate for testing.