Iodine sufficiency and measurements of thyroid function in maternal hypothyroidism

Summary objectives To test the hypothesis that thyroglobulin (Tg) and free T4 (FT4) concentrations more than 2SD from the control mean are not increased in pregnancy in an iodine replete area in the absence of elevated TSH concentrations. The second hypothesis to be tested was that if such abnormalities in FT4 and Tg in the absence of elevated TSH concentrations were to exist they would not be associated with lowered IQs in the progeny. design Cross‐sectional study in New England comparing TSH, Tg, antibodies to Tg and FT4 in volunteer nonpregnant women 20–40 years old with those in hypothyroid mothers and matched euthyroid control mothers. The results are contrasted with those from similar studies reported from iodine deficient areas. subjects Sera obtained at 17 weeks gestation and stored at −20 °C for 8 years were retrieved and analysed from 62 mothers with subclinical hypothyroidism and 124 matched euthyroid mothers. The diagnosis of hypothyroidism was made by finding a TSH concentration > 97·7 percentiile for 25 000 consecutive pregnant women. Sera were also analysed from 53 healthy nonpregnant volunteer women aged 20–40 years. measurements TSH, Tg and Tg antibodies were measured in the sera of the nonpregnant volunteers, and Tg and Tg antibodies in the sera of the pregnant women who had previously been analysed for TSH and FT4. The incidence of FT4 concentrations below the 2·3 percentile of nonpregnant laboratory controls was compared for the euthyroid and hypothyroid mothers and the laboratory normal controls. results Thirty‐one per cent of the 62 hypothyroid mothers had FT4 concentrations below the 2·3 percentile compared with only one (0·8%) of the euthyroid mothers. Mean Tg concentrations did not differ between the nonpregnant controls and the euthyroid pregnant women, 14 ± 10 vs. 16 ± 10 µg/l. Tg concentration in the hypothyroid mothers was 44 ± 61, significantly greater than for either of the euthyroid control groups, P