Determinants of thyroid volume in schoolchildren: fat-free mass versus body fat mass--a cross-sectional study

To investigate the relationship between thyroid volume and body composition in schoolchildren 11 to 15 years of age.

We conducted a cross-sectional study of 126 girls and 86 boys who were living in an urban area and receiving iodine supplementation. The medical history was reviewed. Weight, height, body fat, fat-free mass, and total body water were measured by using a tetrapolar bioelectrical impedance analyzer. Body mass index and body surface area were calculated. Iodine excretion was measured in a morning urine sample by spectrophotometry. Thyroid volume was measured with use of an ultrasound scanner. Thyroid-stimulating hormone was measured from capillary blood samples with use of a neonatal human thyrotropin kit.

All study participants were euthyroid. No study participant had urinary iodine excretion of less than 10 mg/L, and two thirds of the study group had iodine excretion of more than 100 mg/dL. Mean thyroid volume increased from 5.35 +/- 1.11 mL in boys at age 11 years to 8.52 +/- 3.32 mL in boys at age 15 years and from 5.95 +/- 1.70 mL to 7.53 +/- 1.92 mL in girls of corresponding ages. In both sexes, thyroid volume correlated better with height (r = 0.33 in girls and 0.50 in boys), weight (r = 0.35 and 0.43, respectively), and body surface area (r = 0.38 and 0.50, respectively) than with body mass index (r = 0.26 and 0.16, respectively). Thyroid volume showed a significant correlation with fat-free mass (r = 0.39 in girls and 0.49 in boys) and no significant correlation with body fat in both girls and boys.

Thyroid volume is dependent on body size and therefore on growth variables. It depends on fat-free mass as a relatively precise measure of body size and is not related to the fat mass.