Does thyroid function affect fertility?

Hormonal testing that is part of a typical fertility evaluation includes screening for thyroid dysfunction. Overactive or underactive thyroid function can lead to issues with fertility and increased risks in pregnancy, specifically miscarriage.

The typical screening test for thyroid is thyroid-stimulating hormone (TSH) and the results can be confusing since a high TSH is a sign of an underactive thyroid gland.

TSH is the hormone released from the pituitary gland and tells the thyroid to make or not make thyroid hormone. If the thyroid is not making enough thyroid hormone the TSH will be high.

The level of TSH optimal for fertility and in pregnancy can be controversial but the Endocrine Society states higher risk women (those with infertility and recurrent miscarriage) should have a tighter control of thyroid before and during pregnancy (usually <2.5 mIU/L).

Demand on the thyroid increases by 30% in the first trimester of since the baby does not make it’s own thyroid hormones until about 10 weeks of pregnancy.

Hypothyroidism (underactive thyroid, low thyroid, high TSH) has been associated with an increased risk of miscarriage.