National status of testing for hypothyroidism during pregnancy and postpartum

Hypothyroidism, overt or subclinical, is associated with adverse outcomes for pregnant women and their offspring. Knowledge of current national thyroid testing rates and positivity during pregnancy is limited.

The aim of the study was to estimate thyroid testing rate and positivity during pregnancy and postpartum, including testing and positivity rates of thyroperoxidase antibody (TPO Ab) and free T(4) tests in pregnant women with elevated TSH levels (hypothyroid), and in pregnant women having TSH within range (euthyroid).

Records from a large, national sample of pregnant women screened from June 2005 through May 2008 were examined.

The study included 502,036 pregnant women, for whom gestational age information was available.

Testing rates and the prevalence of hypothyroidism during pregnancy and postpartum were measured using assay-specific, trimester-specific reference intervals. Screening and positivity rates of TPO Ab and free T(4) tests were also measured.

Of women ages 18 to 40 yr, 23% (117,892 of 502,036) were tested for gestational hypothyroidism (defined as both subclinical and overt hypothyroidism). Of these, 15.5% (18,291 of 117,892) tested positive for gestational hypothyroidism. Twenty-four percent (22,650 of 93,312) of women with TSH within range and 33% (6,072 of 18,291) of women with elevated TSH were also tested for gestational hypothyroxinemia. Gestational hypothyroxinemia was seen in 0.2% (47 of 22,650) of the tested women with TSH within range and was seen in 2.4% (144 of 6,072) of the tested women having elevated TSH; 0.3% (276 of 93,312) of women with TSH within range received a TPO Ab test, and of these, 15% (41 of 276) tested positive; 0.66% (120 of 18,291) of women with elevated TSH received a TPO Ab test, and of these, 65% (78 of 120) tested positive. Only 20.7% (1873 of 9063) of hypothyroid women received thyroid screening within 6 months postpartum; of these, 11.5% (215 of 1873) were diagnosed with postpartum hypothyroidism.

Gestational hypothyroidism is more common than generally acknowledged. Testing is not common, and test selection is variable. There is a low rate of postpartum follow-up.