The levels of serum thyroxine (T4), triiodothyronine (T3) and protein-bound iodine (PBI) have been determined in 22 clinically euthyroid inhabitants of the goitrous area of Idjwi Island (Republic of Congo) and in 18 normal Belgian adults used as controls. The subjects of the endemic area were selected according to the level of their serum PBI (Group I: 5.4 ± 0.2 (SE) ng/100 ml; Group II: 2.8 ± 0.1 µg/100 ml). The levels of T4 are very close to those of PBI. The mean level of T3 in the Belgian controls is 0.231 ± 0.005 (SE) µg/100 ml. The corresponding values for the African patients of group I and group II are 0.332 ± 0.013 µg/100 ml and 0.655 ± 0.025 µg/100 ml respectively. There is a negative correlation between the levels of T3 and T4 in the serum. The present findings confirm a preferential secretion of T3 by the thyroid in endemic goiter. © 1972 by The Endocrine Society. ; SCOPUS: ar.j ; info:eu-repo/semantics/published