Determinants of thyroid volume as measured by ultrasonography in healthy adults randomly selected

The relationship between thyroid volume and anthropometric characteristics is a matter of controversy. The aim of this study was to investigate thyroid volume and its determinants in healthy adult subjects from a noniodine-deficient area.

Of the 280 000 inhabitants of the city, served by L'Hospitalet de Llobregat, we randomly selected 880 subjects from the census of the city. The participation rate in the study was 44%; after application of several exclusion criteria, a further 28 subjects were excluded because of previously diagnosed thyroid disease. We finally studied 268 subjects representative of the census of the city: 134 male and 134 female, without thyroid disease. We determined the anthropometric characteristics, body mass index, waist-hip ratio, body surface area; body composition by bioelectrical impedance analyser; thyroid volume by ultrasonography; basal TSH, antithyroid antibodies and urinary iodine excretion.

Thyroid volume in our population was higher in males (9.19 ml, CI 9.09-10.65) than in females (6.19 ml, CI 6.02-6.92), P = 0.001. Significant correlations were found among thyroid volume and body weight (r = 0.39, P = 0.0001), height (r = 0.44, P = 0.0001), body mass index (r = 0.13, P = 0.02), waist-hip ratio (r = 0.38, P = 0.0001), body surface area (r = 0.48, P = 0.0001), total body water (r = 0.14, P = 0.02), free fat mass (r = 0.47, P = 0.0001), fat mass (r = 0.37, P = 0.001) and body fat (r = 0.32, P = 0.001). Negative correlation was found between thyroid volume and basal TSH (r = -0.26, P = 0.001). No correlations were found among thyroid volume and iodine excretion, previous pregnancies in women, cigarette smoking and alcohol consumption. In a multiple regression analysis with thyroid volume as the dependent variable, body surface area was demonstrated to account for the 44% of variation of thyroid volume (P = 0.0001).

It is important to know the reference values of the thyroid volume in a population free of iodine deficiency and its determinants. Body surface area accounts for much of the variation of thyroid volume. Age, gender, anthropometric variables, body composition variables and biological variables, do not significantly influence the thyroid volume when considered as possible additions to this baseline model.