The concentrations of triiodothyronine (T3), thyroxine, (T4) and thyroid-stimulating hormone (TSH) were measured by radioimmunoassay (RIA) (T3, TSH) or competitive proteinbinding technique (T4) in the sera of 61 subjects (aged (9-61 yr) from Eastern New Guinea where severe iodine deficiency is known to occur. Only 1 of the subjects was clinically hypothyroid, the remaining 60 being euthyroid, of whom 9 were pregnant females. Excluding the latter from the total group of 61, the remaining 52 subjects comprised 30 goitrous and 22 nongoitrous individuals. Of the 30 subjects with goiter, serum T4 was below normal in 22, whereas only 7 of the 22 nongoitrous subjects had low T4 levels, the majority being within the normal range. The mean serum T4 concentration of the goitrous group (3.4 ± 0.4 μg/100 ml) (mean ± sem) was lower (p < 0.005) than the mean of the nongoitrous group (5.1 ± 0.5 μg/100 ml). By contrast, serum T3 was depressed in only 2 of the goitrous subjects, being normal in 19 and elevated in 9. In the group without thyroid enlargement, T3 was low in 8 of the 22 and normal in the rest. The mean T3 level of 151 ± 12 ng/100 ml (mean ± sem) in the goitrous group was significantly higher than the mean of 99 ± 10 ng/100 ml in the nongoitrous (p < 0.0025). Serum TSH levels showed a pattern which was basically similar in the 2 groups studied. Six subjects were unequivocally hypothyroid biochemically (serum T4 < 2.1 μg/100 ml, T3 < 75 ng/100 ml, TSH > 600 μU/ml) only 1 of whom was judged hypothyroid clinically. It is concluded that relative hypersecretion of T3 occurs in goitrous compared to nongoitrous subjects and may represent an important homeostatic mechanism to combat iodine deficiency. Secondly, hypothyroidism in Eastern New Guinea is probably more common than previously suspected.