The thyroid status of the Yanomamö Indians of southern Venezuela: 1992 update

Previous reports described an apparent paradox in the clinical expression of endemic iodine deficiency in Amazonian Indians, who were severely iodine deficient but had no goiter and were clinically euthyroid. To confirm and explain this unique observation, we estimated the iodine intake and evaluated the functional and structural thyroid status of an isolated population of Yanomamö Indians (n = 104) in southern Venezuela. Twenty-six children, 32 adolescents, and 46 adults were studied with serum thyroid function tests, spot urinary iodide concentration determinations, and sonographic measurement of thyroid gland volume. A subset of adolescents and adults (n = 35) had 24-h fractional thyroidal ¹³¹I uptake determinations. No individual with goiter, clinical hypothyroidism, or cretinism was observed. Children had higher mean serum T₄ (114.5 vs. 102.9 nmol/L; P < 0.02) and serum T₃ (2.96 vs. 2.46 nmol/L; P < 0.02) concentrations than adults, with 2% and 50%, respectively, of children having levels more than 2 SD above the mean for a North American reference population. Serum TSH concentrations were also higher in children than adults (3,3 vs. 2.3 mU/L; P < 0.02), with 25% of the subjects above the reference range. Mean urinary iodide was 61 ± 29 μg/L (range, 10-178 μg/L), reflecting borderline iodine sufficiency. Thyroidal ¹³¹I uptake values were 28 ± 7%, with 33% above the upper limit of normal (30%). Compared to an iodine-replete Swedish population, the thyroid gland volume was above normal (>mean + 2 SD) in 71% of the study children. We conclude that the Yanomamö have borderline iodine deficiency, associated in children with predictably higher serum T₃ and TSH concentrations and a high prevalence of small goiters. The basis for the apparent change in their dietary iodide intake over the past 30 yr is unclear, as is the explanation for their previously postulated resistance to goitrogenesis with more profound iodine deficiency.