Thyroid Function and Pregnancy: Research Review

The thyroid is a butterfly-shaped gland located in the neck. It secretes hormones that regulate metabolism, body temperature, the cardiovascular system, and the digestive system. The thyroid’s proper function is crucial when pregnant because the mother’s thyroid hormones affect fetal growth and brain development.

Thyroid disorders can contribute to pregnancy complications, miscarriage, or a person’s ability to get pregnant. Thyroid disorders sometimes arise from autoimmune problems where a person develops antibodies to specific parts of the thyroid. These tests look for thyroid antibodies as well as measure hormone levels related to healthy thyroid functioning.

Physiology

The thyroid gland, located at the base of your neck, is part of your endocrine system. Thyroid dysfunction is the second most common endocrine disorder after diabetes that affects women of reproductive age¹, among whom 5–15% test positive for thyroid autoantibodies. It is caused by genetic factors and environmental conditions, both of which are yet to be fully understood. 

Hypothyroidism is the most common thyroid alteration observed during pregnancy with the most frequent cause being autoimmune thyroiditis (Hashimoto’s thyroiditis).

Published clinical data

Untreated hypothyroidism can lead to fertility issues with a wide range of adverse outcomes: miscarriage, preterm delivery, gestational hypertension or reduced cognitive function in the offspring². Thyroid autoimmunity with the production of anti-thyroid antibodies (anti-TPO antibodies) may directly attack the fetoplacental unit thus leading to embryo losses or obstetrical complications.

The miscarriage rate in women with subclinical or clinical hypothyroidism at the time of conception, is significantly increased ranging from 60% up to 71.4%³. On the other hand, when adequately treated, no losses were seen and over 90% of patients delivered at term regardless of the severity of the hypothyroidism.

In women inefficiently treated for hypothyroidism⁴, fetal loss rate was 29% compared with 6% in patients whose serum TSH values were restored within the reference range after therapy. Even in least severe case of hypothyroidism with no anti-thyroid antibodies and low TSH levels (between 2.5 and 5 mUI/mL), there is an increased risk of miscarriages that almost doubles as compared with control patients when treatment is not administrated⁵.

Lastly, in a large study including 25 756 women with a singleton pregnancy⁶, pregnancies in women with subclinical hypothyroidism were three times more likely to be complicated by placental abruption and preterm birth as compared to pregnant women with normal thyroid function. These complications were estimated to increase by 60% for every doubling of the serum TSH concentration.

All together, these data showed that screening, monitoring and adequate treatment to restore optimal thyroid function during pregnancy is crucial to minimize risks and lead to term pregnancies with no obstetrical complication in patients affected with hypothyroidism.