TOPLINE

Most untreated pregnant women initially testing positive for subclinical hypothyroidism or hypothyroxinemia do not sustain abnormal thyroid function throughout pregnancy, with more than 88% of those with subclinical hypothyroidism and more than 61% with hypothyroxinemia showing at least one normal test result during gestation. Development of overt hypothyroidism was uncommon in both groups, occurring in approximately 2% of participants.

METHODOLOGY

• Researchers conducted a secondary analysis of two parallel, multicenter, placebo-controlled, randomized trials involving pregnant women with subclinical hypothyroidism (thyroid-stimulating hormone [TSH] ≥ 4.0 mIU/L with normal free thyroxine [FT4] 0.86-1.9 ng/dL) or hypothyroxinemia (TSH 0.08-3.99 mIU/L with FT4 < 0.86 ng/dL) identified before 20 weeks of gestation across 15 sites in the US.
• A total of 580 participants from the placebo groups were included, comprising 326 with subclinical hypothyroidism and 254 with hypothyroxinemia, who underwent monthly TSH and FT4 assessments throughout pregnancy using the ADVIA Centaur assay system.
• Participants with subclinical hypothyroidism had a median screening gestational age of 10.4 weeks compared with 13.1 weeks for the hypothyroxinemia group, with median follow-up of five and four subsequent monthly tests, respectively.
• Outcome measures included the proportion of participants with at least one subsequent normal thyroid function test result during pregnancy, development of overt hypothyroidism, and correlation between thyroid peroxidase (TPO) antibody status and subsequent test results.
• Statistical analysis utilized the Cochran-Armitage trend test to examine associations of subsequent test results with baseline TPO antibody status, and multivariate Cox regression modeling to assess time to first normal thyroid function test result.

TAKEAWAY

• Among participants with subclinical hypothyroidism, 88.7% (289/326) had at least one subsequent normal thyroid function test result during pregnancy compared with 61.4% (156/254) of those with hypothyroxinemia (P < .001).
• Development of overt hypothyroidism during pregnancy was uncommon, occurring in eight of 326 participants (2.5%) in the subclinical hypothyroidism group and five of 254 participants (2.0%) in the hypothyroxinemia group.
• TPO antibody positivity was associated with longer time to first normal thyroid function test result in the subclinical hypothyroidism group (hazard ratio [HR], 0.66; 95% CI, 0.51-0.86; P = .002) but not in the hypothyroxinemia group (HR, 0.92; 95% CI, 0.43-1.98; P = .83).
• In the hypothyroxinemia group, increased maternal age (HR, 0.96 per year; 95% CI, 0.93-0.99; P = .005) and later gestational age at randomization (HR, 0.94 per week; 95% CI, 0.89-0.99; P = .03) were associated with longer time to first normal test result.

IN PRACTICE

“Most untreated pregnant women who initially test positive for subclinical hypothyroidism or hypothyroxinemia do not have sustained abnormal results throughout the remainder of the pregnancy. Strategies to screen and treat these women in early pregnancy may result in the treatment of patients whose test results would spontaneously revert to normal,” wrote the authors of the study.

SOURCE

The study was led by Lauren H. Theilen, MD, MS, University of Utah Health, Salt Lake City. It was published online in Obstetrics & Gynecology.

LIMITATIONS

According to the authors, limitations include the lack of data regarding levels of beta-human chorionic gonadotropin, a pregnancy-related hormone that suppresses TSH through cross-reactivity with TSH receptors, particularly early in pregnancy when beta-human chorionic gonadotropin levels peak. The mean gestational age at screening differed between subclinical hypothyroidism (10.9 weeks) and hypothyroxinemia (13.1 weeks) groups, though this difference is unlikely to have confounded findings given that TSH suppression would be most significant at 8-12 weeks of gestation when beta-human chorionic gonadotropin peaks. The analysis was limited to participants randomly assigned to placebo groups to observe natural history without treatment intervention.

DISCLOSURES

This study received support from grants from the Eunice Kennedy Shriver National Institute of Child Health and Human Development and the National Institute of Neurological Disorders and Stroke. Theilen disclosed receiving support from a grant from the National Heart, Lung, and Blood Institute. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.

This article was created using several editorial tools, including AI, as part of the process. Human editors reviewed this content before publication.