(166) Long-Term Progression to Overt Hypothyroidism Following Subclinical Thyroid Disease Identified During Pregnancy

To determine the incidence and timing of progression to overt hypothyroidism among patients with subclinical hypothyroidism (SCH), thyroid peroxidase (TPO) antibodies identified during pregnancy.

Study Design:

This was a longitudinal cohort study of pregnant patients who underwent thyroid screening before 20 weeks gestation at a large, academic center between 2000 and 2003. Serum thyroid stimulating hormone (TSH) levels were measured, and abnormal results were reflexively tested for free thyroxine (T4); those with both abnormal TSH and free T4 were excluded. SCH was defined as TSH ≥97.5th percentile with free T4 >0.680 ng/dL. TPO antibody positivity was ≥50 IU/mL. Of the 1,186 eligible patients from 2000-2003, 718 had follow-up data. The primary outcome was the development of overt hypothyroidism following the index pregnancy, defined by initiation of thyroid hormone therapy or a clinical diagnosis.

Results:

Over the 25-year study period, 238 (33.1%) patients with subclinical thyroid disease in pregnancy progressed to overt hypothyroidism. The risk was highest in those with both SCH and TPO positivity (52.9%), followed by TPO positivity alone (28.4%) and SCH alone (25.0%, p < 0.001). While most diagnoses occurred more than 5 years postpartum, a notable proportion developed hypothyroidism within the first five years. Higher baseline TSH was associated with earlier progression of overt hypothyroidism (Figure 1).

Conclusion:

One in three individuals with SCH or TPO antibody positivity in pregnancy progressed to overt hypothyroidism within 25 years. The combination of elevated TSH and TPO positivity conferred the greatest risk. These findings underscore the need for long-term follow-up and suggest a potential need to reevaluate current recommendations regarding universal thyroid screening in pregnancy.