Although practice guidelines currently do not recommend thyroid disease screening for women with subfertility or recurrent early pregnancy loss, the authors of a new review article suggest that clinicians should consider doing so. Thyroid disease is the most common endocrine problem found in women of reproductive age, and thyroid function abnormalities, including hyperthyroidism and hypothyroidism, can adversely affect reproductive health by reducing the rate of conception and increasing the rate of early pregnancy loss and adverse pregnancy and neonatal outcomes, according to the review, “Thyroid dysfunction and reproductive health,” which was published in The Obstetrician & Gynaecologist.

“Because function of the thyroid gland is under the control of the hypothalamo-pituitary axis, changes in thyroid function can impact greatly on reproductive function before, during and after conception,” the authors wrote. There is also evidence that autoantibodies play a role in subfertility and early pregnancy loss, even in women who are euthyroid.

Of note to laboratorians, the study finds that based on currently available evidence, “it seems reasonable to measure thyroid function routinely in women presenting with subfertility,” first with a serum thyroid-stimulating hormone (TSH) measurement only. “Given that improvements in reproductive outcomes are seen in those in whom the serum TSH is less than 2.5 mU/l, serum TSH levels should be maintained below 2.5 mU/l for those with both clinical and subclinical hypothyroidism.”

A subclinical hypothyroidism finding (serum TSH of more than 2.5 mU/l with normal free thyroxine (T4)) should urge the clinician to order a repeat serum TSH screening and to check thyroid autoantibodies. “If the serum TSH persists above 2.5 mU/l, treatment with thyroxine replacement therapy (L-T4 ) is warranted to bring the TSH below 2.5 mU/l. The dose of L-T4 should be titrated until the serum TSH is brought to 2.5 mU/l or less, and during this period monitoring of LT4-4 and TSH every six weeks is warranted,” the authors advised. “The evidence is lacking over the benefit of commencing L-T4 in those who are euthyroid with [autoimmune thyroid disease]. It should, however, prompt close monitoring of thyroid function if pregnancy results and monitoring of fetal wellbeing and subsequently neonatal review .”

Miscarriage affects about one in five pregnancies, and recurrent miscarriage, defined as three consecutive miscarriages, affects 1% of couples. “Given that thyroid hormone plays an important part in embryonic development, particularly neurodevelopment, and that until approximately 10 weeks of gestation the fetus is dependent on placental transfer of T4 from the mother, it is unsurprising perhaps that thyroid disease has long been associated with miscarriage,” the authors explained. “Thyroid hormone receptors are also known to be present on the placenta, suggesting that it may have a role in placental development. However, there has previously been insufficient evidence of an association to warrant routine screening of thyroid function in asymptomatic women presenting with recurrent miscarriage.”

Still, thyroid screening in women with a history of recurrent miscarriage is warranted. “Overt thyroid disease should be managed with L-T4 replacement therapy and the serum TSH brought within the target range for pregnancy (that is, at or below 2.5 mU/l),” according to the study. There is insufficient evidence to warrant thyroid replacement in patients with subclinical hypothyroidism and autoimmune thyroid disease.