Iodine deficiency in adults caused preferred synthesis of T3; this observation has not been reported in iodine-deficient hypothyroid newborns. Serum total T4, total T3, and TSH have been determined in nine full term newborns with congenital hypothyroid goiter before and after cutaneous application of iodine. The mothers of these infants had untreated euthyroid goiter and lived during pregnancy in the area of Göttingen, West Germany, known as an iodine-deficient region. Mean total T4 in the newborns was 6.3 +/- 1.6 (mean +/- SD) micrograms/dl compared to 16.6 +/- 3.4 micrograms/dl in normal newborns at 3-4 days of age. Mean T3 in the goitrous newborns was 2.74 +/- 0.66 ng/ml compared to 1.58 +/-0.41 ng/ml in the control group of the same age. Serum TSH remained elevated during the first week of life, with a concentration of 40.9 +/- 28.7 microU/ml (control group, 4.16 +/- 1.43 microU/ml). The cutaneous application of iodine resulted in rapid disappearance of goiter and normalization of T4 and TSH within 5 days. After 30 days of iodine treatment, T3 decreased slowly but remained elevated (2.0 +/- 0.42 ng/ml vs. 1.67 +/- 0.36 ng/ml in the control group). The present findings confirm preferential T3 secretion in newborns with hypothyroid goiter. The goiter is thought to be caused by intrauterine iodine deficiency, because hypothyroid values of T4 and TSH normalized during iodine treatment. General iodine prophylaxis of the population is recommended.