Gestational transient hyperthyroxinaemia (GTH): screening for thyroid function in 23,163 pregnant women using dried blood spots.

OBJECTIVE: Transient elevation of serum free T4 (gestational transient hyperthyroxinaemia; GTH) occurs occasionally during normal pregnancy, especially in early gestation. However, the frequency of GTH and its clinical features remain unclear to date. The aim of this study was to determine the occurrence rate of GTH and the relation between serum levels of hCG, free T4 (fT4), and TSH in a large number of pregnant women.

DESIGN: The four criteria of GTH were as follows: (1) no past history of thyroid disease, (2) negative tests for MCHA and TGHA, (3) no multiple pregnancies or trophoblastic disease and (4) transient hyperthyroxinaemia at less than 16 weeks of gestation. Thyroid function and hCG levels in 23,163 pregnant women were evaluated by mass sreening. If individual fT4 levels were more than the upper limit, blood re-sampling and the clinical and laboratory analysis of thyroid function were performed to exclude women with thyroid disease. The concentrations of hCG, fT4, and TSH in women with GTH and normal pregnant controls (n = 218) were compared. Regression analysis was performed for the comparison between hCG, fT4, and TSH levels in women with GTH.

MEASUREMENTS: Blood samples were obtained using dried blood spots. Blood levels of fT4 was measured by radioimmunoassay, TSH and hCG were measured by fluoroimmunoassay. Anti-microsome antibody (MCHA) and anti-thyroglobulin antibody (TGHA) were measured by indirect agglutination reaction.

RESULTS: GTH was observed in 66 of 23,163 women. The overall occurrence rate of GTH was 0.285%. In 22 of the 66 GTH women, serum TSH was undetectable. Using regression analyses, the concentration of fT4 was correlated with hCG levels in women with GTH (P < 0.05, r = 0.269), whereas the concentration of TSH was not correlated with hCG or fT4 level. The concentrations (M +/- SD) of fT4, TSH, and hCG in women with GTH were 42.5 +/- 12.3 pmol/l, 0.20 +/- 0.31 mU/l and 190.2 +/- 98.8 x 10(3) IU/l, whereas those of controls were 14.6 +/- 3.8 pmol/l, 1.43 +/- 1.25 mU/l and 60.1 +/- 45.1 x 10(3) IU/l. The concentrations of fT4 and hCG were significantly (P < 0.0001) higher than those of normal controls, and TSH was significantly (P < 0.0001) lower than those of normal controls.

CONCLUSION: The occurrence rate of gestational transient hyperthyroxinaemia was 0.285%, and could possibly be attributed to increased levels of circulating hCG. Based on the data obtained from a large number of pregnant women, we propose gestational transient hyperthyroxinaemia as a definite clinical entity.