Thyroid disease and fertility

Thyroid hormones act on almost every tissue, including the ovary, the womb lining and the pituitary gland that drives the menstrual cycle. When levels drift out of range, cycles can become irregular, heavier, lighter or stop altogether, and ovulation can become unpredictable. The NHS lists irregular periods among the symptoms of an underactive thyroid.

Because those changes overlap with other causes such as polycystic ovary syndrome, stress, low weight or early menopause, thyroid tests are a standard early step whenever periods are irregular or conception is delayed.

Typical features are tiredness, weight gain, feeling cold, constipation, low mood, dry skin and hair, and irregular periods, and they usually develop slowly. In the UK the commonest cause is Hashimoto disease, an autoimmune condition, and it most often affects women between 30 and 50 according to the NHS.

Treatment is with levothyroxine tablets, usually lifelong, with blood tests to fine-tune the dose. For someone trying to conceive, the aim is to have thyroid hormone levels well controlled before and during pregnancy, so a planned review with a GP or endocrinologist is worthwhile. Early pregnancy often increases the dose needed, so clinicians commonly check levels soon after a positive test.

Excess thyroid hormone can cause anxiety, tremor, heat intolerance, a fast or irregular heartbeat, weight loss, and changes in periods. The NHS says it is about ten times more common in women and typically begins between 20 and 40, with Graves disease the leading cause.

Treatment may be medicines to reduce hormone production, radioiodine, or surgery. Each has implications for pregnancy: some anti-thyroid medicines are changed or avoided around conception and in the first trimester, and radioiodine is not given to anyone who is pregnant and is followed by a period during which pregnancy is postponed. The specific advice comes from the treating team, which is why the NHS stresses speaking to a specialist before trying.

Some people have thyroid antibodies without abnormal hormone levels, and others have a slightly raised TSH with a normal T4, often called subclinical hypothyroidism. Whether these findings affect fertility or miscarriage risk, and when treatment helps, is an area where guidance is cautious and specialists differ in practice. A sensible approach is to repeat borderline results rather than act on one reading, and to have the decision made by someone who knows your history.

Men are not exempt. Thyroid disorders can alter testosterone-binding proteins, libido and sperm quality in some cases. This is less commonly the main explanation than in women, but a thyroid blood test is reasonable when a man has unexplained low sperm quality along with symptoms such as fatigue or weight change.

A clinician will usually ask about symptoms, family history of thyroid or autoimmune conditions, current medicines (amiodarone and lithium, for example, can affect the gland), and plans for pregnancy. Blood tests follow, and thyroid antibodies may be added.

• Check TSH and T4 before trying to conceive when you know you have thyroid disease.

• Do not stop or change levothyroxine or anti-thyroid medicine on your own.

• Ask whether your medicine is suitable in pregnancy and when to retest.

• Tell the fertility clinic about any thyroid condition, since it affects their planning.

Palpitations with chest pain, severe agitation, fever and confusion in someone with an overactive thyroid may signal thyroid storm and need emergency care through 999. Extreme drowsiness, a very slow heartbeat or low body temperature with known hypothyroidism also needs urgent assessment.