Postpartum thyroid dysfunction in pregnant thyroid peroxidase antibody-positive women living in an area with mild to moderate iodine deficiency: is iodine supplementation safe?

In moderately iodine-deficient, pregnant, thyroid peroxidase antibody (TPO-Ab)-positive women the role of iodine supplementation in the development of postpartum thyroid dysfunction (PPTD) was studied in a placebo-controlled, randomized, double blind trial. Screening for TPO-Ab was performed in early pregnancy in a population of healthy pregnant Danish women with no previous diagnosed thyroid disease (prevalence, 117 of 1,284; 9.1%). The participants were randomized, stratified according to TPO-Ab level, to three groups. All participants received a daily vitamin and mineral tablet with 150 microg iodine or no iodine. The +/+ group received iodine during pregnancy and the postpartum period, the +/- group received iodine during pregnancy only, and the -/- group received no iodine supplementation. A total of 66 TPO-Ab positive women were followed, and in the postpartum period sera were collected at 8-week interval for biochemical evaluation of thyroid function and antibody level. Compliance was evaluated by 24-h urinary iodine measurements. PPTD developed in 55% of the participants. In 67% of the cases abnormal TSH was accompanied by abnormalities in thyroid hormones, whereas 33% had abnormal serum TSH only. There was no statistically significant difference in the frequency of PPTD in the three groups: +/+ group, 59% (95% confidence interval, 36-79%); +/- group, 60% (36-81%); and -/- group, 46% (26-67%). There were also no differences in the severity of the PPTD, as evaluated by duration and grade of deviation of TSH and thyroid hormones from normality. The occurrence, severity, and type of PPTD predominantly depended on the TPO-Ab level: TPO-Ab below 200 U/L at screening, 35% developed PPTD; TPO-Ab of 200-900 U/L, 54%; and TPO-Ab above 900 U/L, 75% developed PPTD. Women with low levels of antibodies predominantly remained euthyroid or had hyperthyroidism only, whereas women with high antibody levels had hyperthyroidism followed by hypothyroidism or hypothyroidism only. We conclude that iodine supplementation (150 microg) during pregnancy and the postpartum period to TPO-Ab-positive women living in an area with mild to moderate iodine deficiency did not induce or worsen PPTD. The study confirmed that screening for TPO-Ab in early pregnancy can predict women at high risk for development of PPTD.