From hyperthyroidism to hypothyroidism, strategies for coping with thyroid disease during pregnancy

Author: Yang Zhaojun, Chief Physician of China-Japan Friendship Hospital

Reviewer: Wu Xueyan, Chief Physician, Peking Union Medical College Hospital

During pregnancy, women's hormone levels change significantly, especially human chorionic gonadotropin (HCG) and estrogen. The fluctuation of these hormones not only directly affects the maintenance and development of pregnancy, but also may indirectly affect thyroid function, thereby causing or aggravating thyroid disease. At the same time, the presence of thyroid disease will in turn affect the smooth progress of pregnancy and increase the risk of maternal and fetal complications.

Therefore, it is crucial to understand and effectively manage thyroid disease during pregnancy.

1. Impact and management of thyroid disease during pregnancy

During pregnancy, changes in maternal hormone levels can have a significant impact on thyroid function. In early pregnancy, rising levels of human chorionic gonadotropin (HCG) increase thyroid hormone secretion, which may cause women with pre-existing overactive thyroid (hyperthyroidism) to become more ill. However, after the second trimester, symptoms of autoimmune thyroid diseases such as hyperthyroidism tend to ease as the immune system adjusts to the presence of the fetus.

In contrast, for patients with underactive thyroid gland (hypothyroidism), changes in HCG or estrogen levels during pregnancy can lead to an increase in thyroid reserve, which may necessitate an increase in the dose of levothyroxine (such as Euthyrox) to maintain stable thyroid function.

For patients with hyperthyroidism, especially during pregnancy, antithyroid drug treatment is the most commonly used method. The first choice drug is propylthiouracil, because it rarely crosses the placental barrier and has less impact on the fetus. It is currently the first choice antithyroid drug recommended by most countries for early pregnancy (within 12 weeks of pregnancy). Although methimazole is effective, it can cross the placenta and is associated with adverse reactions such as neonatal scalp defects in a few cases, so it is usually used as a second-line option. If drug treatment is ineffective or serious adverse reactions occur, surgical treatment should be considered. The timing of surgery is usually chosen in the second trimester to reduce the impact on mother and baby.

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Patients with hypothyroidism must continue to take levothyroxine during pregnancy to ensure adequate thyroid hormone levels to support the normal development of the fetal nervous system and brain. Appropriate dose adjustment is essential to prevent adverse pregnancy outcomes.

II. Treatment of subclinical hypothyroidism and management of postpartum thyroid disease

Subclinical hypothyroidism refers to a state in which thyroid function is slightly decreased, manifested as elevated thyroid stimulating hormone (TSH) levels and normal free thyroxine (FT4) levels. For patients with subclinical hypothyroidism with TSH>10mIU/L during pregnancy, levothyroxine sodium replacement therapy is required according to clinical hypothyroidism, but there is some controversy as to whether patients with subclinical hypothyroidism with TSH<10mIU/L need treatment. Although some studies have shown that this type of subclinical hypothyroidism may be related to the intellectual development of offspring, there is no conclusive evidence that treatment of all patients with subclinical hypothyroidism can bring clear benefits. Therefore, treatment strategies vary depending on individual differences. The US guidelines recommend that pregnant women with subclinical hypothyroidism who are positive for thyroid peroxidase antibodies (TPOAb) should be treated with levothyroxine, while those with negative TPOAb should be treated based on specific circumstances.

In the postpartum period, for patients with subclinical hypothyroidism who have received thyroid hormone replacement therapy during pregnancy, the medication should be stopped immediately after delivery, and the thyroid function should be rechecked 4 to 6 weeks later. If the thyroid function returns to normal at this time, the medication can continue to be stopped; otherwise, the treatment plan needs to be re-evaluated. For women who have hypothyroidism before pregnancy and need to take thyroid hormones for a long time, the drug dosage should be restored to the pre-pregnancy level after delivery, and the thyroid function should be checked again 4 to 6 weeks after delivery to determine whether further dosage adjustment is needed.

3. Lactation management of patients with hyperthyroidism

For women with hyperthyroidism, safety is the focus of attention when continuing drug treatment during breastfeeding. Both propylthiouracil and methimazole can be secreted into breast milk in very small amounts, with the former secreting less. Existing evidence shows that both have no significant effect on infant thyroid function within a small to moderate daily dose range. Therefore, the latest domestic and international guidelines do not clearly recommend which antithyroid drug is preferred during breastfeeding, but the minimum dose principle should be followed. It is recommended to take the drug immediately after breastfeeding and wait at least 3 to 4 hours before the next breastfeeding to reduce the chance of infant exposure to the drug.

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