"The findings of our study suggest that the majority of cases of subclinical hypothyroidism and isolated maternal hypothyroxinaemia diagnosed during pregnancy recover after delivery," said senior author Bijay Vaidya, PhD, Consultant Endocrinologist and Honorary Associate Professor, Royal Devon and Exeter Hospital, University of Exeter Medical School, Exeter, UK. "Therefore, most women starting L-thyroxine in pregnancy for these conditions may be able to stop the drug after pregnancy. Reassessing the indication for L-thyroxine outside of pregnancy in these women will prevent women from unnecessarily remaining on the drug long-term," Dr. Vaidya said.
The Endocrine Society guidelines recommend L-thyroxine treatment for all pregnant women with subclinical hypothyroidism and The American Thyroid Association guidelines recommend this treatment for pregnant women with both subclinical hypothyroidism and detectable thyroid-peroxidase antibodies. However, the recommended duration of treatment is unclear.
The authors examined data from 523 healthy pregnant women with no known history of thyroid disorder. Of this group, 65 women (12.4%) had subclinical hypothyroidism (thyroid stimulating hormone [TSH] level > 3 mIU/L measured at 28 weeks gestation). When the women were reevaluated approximately 5 years later, normal thyroid function was found in 49 of the 65 women (75.4%) who had subclinical hypothyroidism during pregnancy.
Predictors of Persistent Hypothyroidism Identified
Significant risk factors for persistent abnormal TSH levels postpregnancy were the presence of thyroid peroxidase antibodies during pregnancy and a TSH level >5 mIU/L during pregnancy.
"These data suggest that if a pregnant woman is found to have a TSH >3 at 28 weeks of pregnancy, she is at high risk (25.6%) of having persistent hypothyroidism when checked a mean of 4.9 years [after pregnancy]," commented Stephanie L. Lee, MD, PhD, Associate Professor of Medicine, Director of the Thyroid Health Center, in the Section of Endocrinology, Diabetes and Nutrition at Boston Medical Center. "The significance of this risk can be determined by comparing what the prevalence of an elevated TSH in all women of the childbearing age, which is only 2% to 3%. The risk of persistent hypothyroidism is even more profound in women with positive thyroid antibodies (~85% had high TSH levels). Thyroid antibodies are found in most patients with Hashimoto's thyroiditis, the most common cause of hypothryoidsim in adults," said Dr. Lee, a member of The Endocrine Society Guidelines committee who approved the clinical practice guidelines on management of thyroid dysfunction during pregnancy and postpartum.
"In our study, pregnant women with subclinical hypothyroidism, who are negative for thyroid peroxidase antibodies or have serum thyrotropin level below 5 mIU/L, are more likely to have normal thyroid function after pregnancy," Dr. Vaidya said. "We suggest these women can stop L-thyroxine soon after delivery. If pregnant women are treated with L-thyroxine for isolated maternal hypothyroxinaemia, they too can stop the drug after delivery. We suggest these women, who stop L-thyroxine after delivery, should have an assessment of thyroid function about 6 weeks after the delivery," Dr. Vaidya added.
Dr. Lee commented that "although the authors suggest that 'only' 25.6% of women have a persistent elevation of TSH (and hypothyroidism) at an average of 4.9 years after pregnancy, I would interpret that as saying that women with a TSH >3 during pregnant who have positive antibodies should remain on thyroid hormone because of a very high risk (85%) of persistent elevated TSH. Thus, women with a TSH >3 during pregnancy should be monitored carefully and if additional risks for hypothyroidism are found—such as other autoimmune disease in the patient, a family history of autoimmune disease including thyroid dysfunction or evidence of chronic thyroiditis on thyroid ultrasound—the patient also should be continued on hormone therapy," Dr. Lee said. She added that women who stop taking L-thyroxine treatment after delivery should be monitored for elevated TSH levels approximately 4 to 6 weeks after cessation of treatment and at least annually thereafter.
Shields BM, Knight BA, Hill AV, Hattersley AT, Vaidya B. Five-year follow-up for women with subclinical hypothyroidism in pregnancy. J Clin Endocrinol Metab. 2013;98(12):E1941-1945.