BACKGROUND
Thyroid hormone is critical for normal development of the baby, especially in early stages of pregnancy when the baby entirely depends on thyroid hormone coming from mother. Hypothyroidism (low thyroid hormone levels) in pregnancy has been associated with several problems during pregnancy, such as miscarriage, early delivery, and problems with baby’s brain development. Hypothyroidism due to Hashimoto’s thyroiditis is the most common endocrine disorders affecting pregnant women in developed nations. In women without thyroid disease, thyroid hormone production naturally increases during pregnancy to provide enough thyroid hormone for mother and baby. Women who are already taking levothyroxine for hypothyroidism usually require higher dose of levothyroxine during pregnancy. Women with Hashimoto’s thyroiditis who do not require levothyroxine before pregnancy may need levothyroxine during pregnancy, because thyroid hormone production may not increase appropriately due to interference from high levels of thyroid antibody.

Currently, the American Thyroid Association (ATA) recommends keeping blood TSH (thyroid stimulating hormone) levels below 2.5mIU/L during pregnancy in patients who require levothyroxine treatment. For those who are not on levothyroxine before pregnancy, the ATA recommends considering starting levothyroxine for TSH over 4mIU/L, especially if patient has high thyroid antibody levels. Generally, patients are advised to increase their levothyroxine dose by 2 pills a week once pregnant if they were taking levothyroxine before pregnancy. However, it may be too much for some patients. Currently, there is no clear way to tell which patients would require the dose increase and which patients would not. The researchers of this study aimed to determine how many patients with Hashimoto’s thyroiditis develop hypothyroidism in early pregnancy, and what pre-pregnancy TSH level would indicate that patients need to increase or start levothyroxine once pregnant.

THE FULL ARTICLE TITLE
Moleti M et al Preconception thyrotropin levels and thyroid function at early gestation in women with Hashimoto’s thyroiditis. J Clin Endocrinol Metab. Epub 2023 Jan 9. PMID: 36620924

SUMMARY OF THE STUDY
The researchers studied 260 women with Hashimoto’s thyroiditis who were seen in a single hospital in Italy between 2008-2017. All women had TSH ≤ 2.5mIU/L within 6 months of pregnancy and had TSH measured at the time of pregnancy and every 4-6 weeks afterwards during pregnancy.

Among these women, 138 patients had normal thyroid levels without levothyroxine and 122 patients were taking levothyroxine for hypothyroidism before pregnancy. About 30% of these women developed TSH above 2.5mIU/L at their first prenatal visit. However, more women who were not taking levothyroxine before pregnancy developed TSH over 4mIU/L compared to women who were taking levothyroxine before pregnancy (20% vs 10%). The researchers found that women with hypothyroidism from Hashimoto’s thyroiditis on levothyroxine pre-pregnancy were 16-times more likely to have TSH >2.5mIU/L if their pre-pregnancy TSH was above 1.24mIU/L. For women with Hashimoto’s thyroiditis but not on levothyroxine before pregnancy, those who had pre-pregnancy TSH above 1.73mIU/L were 16-times more likely to develop TSH > 2.5mIU/L in early pregnancy, and those who had pre-pregnancy TSH above 2.07mIU/L were 17-times more likely to develop TSH > 4mIU/L in early pregnancy.

WHAT ARE THE IMPLICATIONS OF THIS STUDY?
The authors concluded that women with Hashimoto’s thyroiditis require pre-pregnancy TSH levels to be 30-50% lower than 2.5mIU/L to maintain normal thyroid levels in early pregnancy. Although the current ATA guidelines discuss TSH goals for patients who are pregnant, there is no clear guidelines on what pre-pregnancy TSH should be to ensure normal thyroid function in early pregnancy. Early pregnancy is an important period where a lot of development occurs, but we do not currently have enough information to set pre-pregnancy TSH targets to ensure normal thyroid function in early pregnancy in those at risk of thyroid dysfunction. Results of this study give us potential pre-pregnancy TSH targets for women who have Hashimoto’s thyroiditis and are considering pregnancy in near future. It also emphasizes need for further studies in the care of pregnant women with hypothyroidism starting pre-pregnancy.

— Sun Y. Lee, MD, MSc

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