Differences in Diagnostic Criteria Mask the True Prevalence of Thyroid Disease in Pregnancy: A Systematic Review and Meta-Analysis.

BACKGROUND: The reported prevalence of thyroid disease in pregnancy varies widely through the published literature. These discrepancies are due to differences in criteria for euthyroidism, nationality, iodine status, and gestational age at screening. As a result, currently, an accepted rate of prevalence does not exist for the various thyroid diseases in pregnancy. Understanding the true prevalence rates of these disorders has important implications for clinical management and the ongoing discussion regarding universal screening. The aims of this study were to assess (i) the true prevalence of thyroid disorders in pregnancy and (ii) the impact of diagnostic methodology on these rates.

METHODS: A systematic review was conducted of the existing literature, including the Pubmed database and references from relevant review articles. Sixty-three studies reporting prevalence of overt hypothyroidism, subclinical hypothyroidism, isolated hypothyroxinemia, subclinical hyperthyroidism, and overt hyperthyroidism in pregnant women were included. Studies were further classified by thyrotropin (TSH) cutoff for diagnosis in hypothyroid disease and timing of screening for hyperthyroid disease. Meta-analysis yielded pooled prevalence rates, with subgroup analyses for TSH cutoff and timing of screening. Analysis of studies using the 97.5th percentile TSH cutoff was assessed to yield the most accurate prevalence rates for hypothyroidism.

RESULTS: Pooled prevalence rates for hypothyroidism calculated from studies using the 97.5th percentile as an upper limit for TSH were 0.50% for overt hypothyroidism, 3.47% for subclinical hypothyroidism, and 2.05% for isolated hypothyroxinemia. Pooled prevalence rates in the first and second trimesters for hyperthyroidism were 0.91% and 0.65%, respectively, for overt hyperthyroidism and 2.18% and 0.98%, respectively, for subclinical hyperthyroidism.

CONCLUSION: Population-based, trimester-specific TSH cutoffs for diagnosis of hypothyroid disease in pregnancy result in more accurate diagnosis and better estimates for prevalence of disease. Prevalence of hyperthyroidism in pregnancy varies depending on timing of screening. The prevalence rates reported in this study represent the best estimate to date of the true rates of thyroid disease in pregnancy.