Thyroid hormone influences the hormones that control the menstrual cycle. When thyroid function is abnormal, periods can become heavier, lighter, irregular or absent — and fertility can be affected.
How the thyroid affects periods
The thyroid interacts with the hypothalamus, pituitary and ovaries. It influences sex hormone binding globulin, prolactin and the balance of oestrogen and progesterone.
- Hypothyroidism often causes heavy, prolonged or irregular periods, and can contribute to infertility and miscarriage.
- Hyperthyroidism often causes lighter, shorter or absent periods.
Heavy periods
Heavy menstrual bleeding is a common feature of hypothyroidism. It can also cause iron-deficiency anaemia, which adds fatigue. Treating the thyroid often improves flow over a few cycles.
Heavy bleeding has many other causes — fibroids, polyps, adenomyosis, bleeding disorders — so persistent symptoms need gynaecological assessment.
Irregular or absent periods
Both thyroid extremes can disrupt ovulation, leading to irregular cycles or missed periods. In hyperthyroidism, periods may become very light or stop.
Fertility
Thyroid dysfunction can affect ovulation and early pregnancy. Guidelines recommend optimising thyroid function before conception, especially in people with known thyroid disease.
If you are trying to conceive and have irregular periods, fatigue, weight change or a family history of thyroid disease, thyroid testing is reasonable.
Pregnancy
Thyroid requirements increase in pregnancy, and untreated thyroid disease can affect both mother and baby. Monitoring is more frequent, and doses are adjusted.
Menopause
Menopause and thyroid disease can overlap in symptoms — fatigue, sleep disturbance, mood changes and weight gain. Thyroid testing helps distinguish them. Hypothyroidism can also worsen menopausal symptoms.
What to do
- Track your cycle and symptoms.
- Ask for thyroid testing if periods change alongside other thyroid symptoms.
- Correct thyroid function before trying to conceive if possible.
- Treat iron deficiency if present.
- Seek gynaecological assessment for persistent heavy or irregular bleeding.
The bottom line
Thyroid disease is a treatable contributor to menstrual and fertility problems. If your cycle has changed alongside fatigue, weight change or temperature intolerance, a simple thyroid test is a reasonable first step.