In 14 subjects from a region of endemic goiter, with palpable but not visible thyroid glands, the thyroid Il31 clearance was on the average 108.4 ml/m (range 38.5-168.2); renal I131 clearance was 37.1 ml/m (range 8.0-72.0); urinary Il27 excretion 22.5 [mu]g/day; daily rate of hormonal I secretion 22.5 [mu]g/d (range 6.8-37.1). In 12 patients, conversion ratios averaged 78% (range 42-100%) and 24 hour protein-bound I131 .081% of dose/1 of plasma (range: .011-.522). In 20 subjects, salivary radioactivity 24 hours after the administration of 50 [mu]c of I131 averaged 1341 c/m/3cc (range: 91-4670). Thus in all respects, except for the daily rate of hormonal I secretion and the total urinary stable I the glands from these subjects behaved as in thyrotoxicosis. These findings are best explained by assuming a chronic I deficiency.