BACKGROUND
Iodine is essential for the thyroid gland to make thyroid hormone as the thyroid hormones contain iodine. During pregnancy, requirements for thyroid hormone increase. Indeed, thyroid hormone levels often increase by nearly 50% with an associated increased daily iodine requirement of 50%. Further, the thyroid gland’s size increases by 10% in countries that have plenty of iodine in their diet (such as the US) and by 20 to 40% in areas of iodine deficiency. Maintaining adequate iodine intake is essential for thyroid hormone production, which plays a vital role in the development of the baby. Therefore, for pregnant women, the World Health Organization recommends a daily intake of 250 μg of iodine. The American Thyroid Association (ATA) recommends adding to the diet a daily oral supplement that contains 150 μg of iodine in planning for pregnancy, during pregnancy, and during the postpartum period to achieve this.

Hashimoto’s thyroiditis is the most common cause of hypothyroidism and is common in women during childbearing years. Hashimoto’s thyroiditis is an autoimmune condition characterized by thyroid peroxidase (TPO) antibodies. Iodine can variably affect thyroid function in patients with Hashimoto’s thyroiditis, often worsening the hypothyroidism and, rarely, causing hyperthyroidism. Because of this, there is a concern that iodine supplementation in patients with Hashimoto’s thyroiditis during pregnancy could potentially lead to adverse effects. This study aimed to examine the effects iodine supplementation thyroid hormone levels and TPO antibodies in pregnant women with preexisting Hashimoto’s thyroiditis.

THE FULL ARTICLE TITLE
van Heek L et al 2021 Avoidance of iodine deficiency/ excess during pregnancy in Hashimoto’s thyroiditis. Nuklearmedizin. Epub 2021 Mar 23. PMID: 33759148.

SUMMARY OF THE STUDY
This study included 20 adult pregnant women with Hashimoto’s thyroiditis who were treated at the University Hospital of Cologne, Germany, between December 1, 2012, and December 1, 2014. These patients were given iodine supplementation during pregnancy and had a serum thyroid peroxidase (TPO) antibody titer >35 IU/ml at the onset of pregnancy and at least two measurements of TPO antibody levels during pregnancy and once after pregnancy. Measurements of serum thyroid stimulating hormone (TSH) and TPO antibody levels and levothyroxine dose requirements were recorded.

Of the 20 patients, 18 were already on levothyroxine therapy for hypothyroidism. During the course of pregnancy, the levothyroxine dose was increased in 10 patients, reduced in 4, and held constant in 6. As compared with the beginning of pregnancy, TSH levels decreased by the end of pregnancy. From the onset to the 20th week of pregnancy, only 2 patients had TSH levels outside the reference range (1 below and 1 above). By the end of pregnancy, only 1 patient had a TSH below the reference range.

During early pregnancy, the average TPO antibody level was 411±335 IU/ml and at the end of pregnancy, it was 137±214 IU/mL. Serum TPO antibody levels decreased in 18 patients during pregnancy. However, in 1 patient, TPO antibodies increased from 60 IU/mL to 237 IU/ mL, while in another patient, levels remained constant at 1000 IU/mL. After pregnancy, the TPO antibody levels were lower than at the beginning of pregnancy in 17 of the 20 patients, while remaining constant in 1 and increasing in 2 patients. In 2 of the 20 patients evaluated, serum TPO antibody levels at the end of pregnancy were negative.

WHAT ARE THE IMPLICATIONS OF THIS STUDY?
This study shows that iodine supplementation during pregnancy did not significantly affect either thyroid hormone or TPO antibody levels in women with Hashimoto’s thyroiditis. While this was a small study, these results suggest that routine iodine supplementation in the doses recommended by national guidelines may be safely given to pregnant women with preexisting Hashimoto’s thyroiditis.

— Alan. P. Farwell, MD

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