Thyroid and fertility: a small factor that matters

The hormones produced by the thyroid gland affect many body processes, including those linked to reproduction. When the thyroid works slowly, a condition called hypothyroidism, the menstrual cycle can become irregular, ovulation can be disrupted, and the lining of the womb may not prepare properly. Even a mild disorder, where only the TSH is slightly raised while the other hormones are normal, can affect some women, especially when thyroid antibodies are also present.

An overactive thyroid, or hyperthyroidism, can also disturb the cycle and affect fertility, but it is rarer in this context. Importantly, these disorders are often silent; a woman may feel nothing unusual and still have a thyroid worth checking and correcting.

The initial check is simple: a blood test for TSH, the hormone that regulates the thyroid. If it is outside the normal range, further measurements are added, such as the free thyroid hormones and, when needed, antibodies against the thyroid, which indicate an autoimmune tendency. Together these clarify the picture and help decide whether treatment is needed.

For women planning pregnancy or undergoing fertility treatment, many guidelines recommend keeping TSH within an appropriate range, because needs change once pregnancy begins. Checking the thyroid is therefore a natural part of the pre-conception assessment, not an unnecessary step.

In the first weeks of pregnancy, the baby depends entirely on the mother's thyroid hormones, because its own gland has not yet started to work. For this reason the need for thyroid hormone rises early, and a woman already taking thyroid medication often needs a higher dose as soon as pregnancy is confirmed. An untreated underactive thyroid in this period is linked to a higher risk of miscarriage and effects on development.

This is why the thyroid is checked early and followed during pregnancy when there is a disorder. Dose adjustment is done quickly and with repeated tests, so the level stays where it should. For most women this is a simple follow-up that gives great reassurance.

If you are trying to conceive, a thyroid check is a simple and worthwhile step, especially if you have irregular cycles, a history of miscarriage, or a family history of thyroid disease. If a disorder is found, treatment is often easy and improves chances. If everything is normal, that is one less obstacle to worry about.

What does not help is taking thyroid medication without a proven disorder, or changing the dose without tests. The thyroid is a factor adjusted carefully and by measurement, not by guesswork. Your team will place the thyroid in the full context of your assessment.