Progression of Gestational Subclinical Hypothyroidism and Hypothyroxinemia to Overt Hypothyroidism After Pregnancy: Pooled Analysis of Data from Two Randomized Controlled Trials

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Abstract
Background: To examine the incidence of overt hypothyroidism 1 and 5 years after pregnancies where screening before 21 weeks identified subclinical hypothyroidism (SH) or hypothyroxinemia (HT). Methods: Secondary analysis of two multicenter treatment trials for either SH or HT diagnosed between 8 and 20 weeks gestation. Current analyses focus only on individuals randomized to the placebo groups in the two parallel studies. SH was diagnosed with thyrotropin (TSH) ≥4.0 mU/L and normal free T4 (fT4) (0.86–1.9 ng/dL). HT was diagnosed with normal TSH (0.08–3.99 mU/L) but fT4 p < 0.001) and year 5 (15.6% vs. 2.6%, p < 0.001) for participants with SH compared with those with HT. This progression was more common in individuals with TSH values >10 mIU/mL. Baseline TPO level >50 IU/mL in participants with SH was associated with higher rates of hypothyroidism at year 1 (26.7% vs. 6.5%, odds ratio [OR] = 5.3 [confidence interval (CI) 2.6–10.7]) and year 5 (30.5% vs. 7.5%, OR = 5.4 [CI: 2.8–10.6]) compared with those with TPO levels ≤50 IU/mL. For participants with HT, no differences in overt hypothyroidism were seen at 1 year related to baseline TPO level >50 IU/mL (1/10 (10%) vs. 6/218 (2.8%), OR = 3.9 [CI: 0.43–36.1]), but more participants with TPO levels >50 IU/mL developed hypothyroidism by year 5 (2/10 (20%) vs. 4/218 (1.8%), OR = 13.4 [CI: 2.1–84.1]). Conclusion: SH is associated with higher rates of overt hypothyroidism or thyroid replacement therapy within 5 years of delivery than is HT when these conditions are diagnosed in the first half of pregnancy.

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