Determinants of thyroid volume in healthy French adults participating in the SU.VI.MAX cohort

OBJECTIVE To study the relative importance of determinants of thyroid volum e. DESIGN Cross-sectional study on a sample of subjects issued from the SU.VI. MAX cohort. SUBJECTS 2987 French subjects (1713 women aged 35-60 years and 1274 men age d 45-60 years). None of them had previous or present thyroid disease. MEASUREMENTS Thyroid volume was determined by ultrasound. Serum TSH and fre e thyroxine (fT(4)) were measured in duplicate. Urinary iodine and urinary thiocyanate were assayed in random morning urine samples. RESULTS For both sexes, thyroid volume (ml) was positively correlated with weight, height, body mass index and body surface area (P = 0.0001) and nega tively with age for females (P = 0.0009). When the urinary iodine concentra tion was adjusted for urinary thiocyanate concentration and their interacti on, the thyroid volume was negatively correlated with urinary iodine (males P = 0.02, females P = 0.006) and positively correlated with urinary thiocy anate (males P = 0.0001, females P = 0.004). Mean thyroid volume was greate r among active smokers than non-smokers (males P < 0.0001, females P = 0.00 04) and was greater among former smokers than among non-smokers (males P = 0.0001, females = 0.004). Free T-4 and thyroid volume were positively corre lated for both sexes (P = 0.0001). TSH was negatively correlated with thyro id volume for both groups (P = 0.0001). Female users of oral contraception (aged 35-45 years) had a smaller thyroid volume than non-users (P = 0.0009) . CONCLUSIONS The state of borderline iodine deficiency observed in France, i n association with a slightly goitrogenic environment, may result in sustai ned stimulation of the thyroid, independently of TSH level, and is of param ount importance in the formation of goitre. Smoking may affect the thyroid, inducing marked long-lasting thyroid enlargement.