How the thyroid affects your cycle
Thyroid hormones interact with the hormones that run your cycle (estrogen, progesterone, prolactin). When thyroid levels are too low or too high, ovulation and the timing of your period can shift. Both an underactive and an overactive thyroid can disrupt periods.
Underactive thyroid (hypothyroidism)
Too little thyroid hormone. Cycles often become heavier, longer, or more frequent, and you may have fatigue, weight gain, feeling cold, dry skin, hair thinning, and low mood. It can also raise prolactin, which suppresses ovulation.
Overactive thyroid (hyperthyroidism)
Too much thyroid hormone. Cycles often become lighter, shorter, or absent, with symptoms like weight loss, rapid heartbeat, anxiety, shakiness, and feeling hot.
When to get tested
A simple blood test (TSH, often with free T4) checks thyroid function. Ask for it if you have unexplained irregular/heavy/missing periods plus any thyroid symptoms, a family history of thyroid disease, or you're trying to conceive. Thyroid problems are very treatable - and treatment usually brings the cycle back to normal.
Thyroid, fertility and the future
Untreated thyroid issues can make it harder to conceive and raise pregnancy risks, which is why doctors often check thyroid function during fertility work-ups. If you're diagnosed, treatment is straightforward (usually a daily tablet for hypothyroidism) and monitored with periodic blood tests.