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 volume.

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 aged 45-60 years). None of them had previous or present thyroid disease.

Measurements: Thyroid volume was determined by ultrasound. Serum TSH and free thyroxine (fT4) 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 negatively with age for females (P = 0.0009). When the urinary iodine concentration was adjusted for urinary thiocyanate concentration and their interaction, the thyroid volume was negatively correlated with urinary iodine (males P = 0.02, females P = 0.006) and positively correlated with urinary thiocyanate (males P = 0.0001, females P = 0.004). Mean thyroid volume was greater among active smokers than non-smokers (males P < 0.0001, females P = 0.0004) and was greater among former smokers than among non-smokers (males P = 0.0001, females = 0.004). Free T4 and thyroid volume were positively correlated for both sexes (P = 0. 0001). TSH was negatively correlated with thyroid 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, in association with a slightly goitrogenic environment, may result in sustained stimulation of the thyroid, independently of TSH level, and is of paramount importance in the formation of goitre. Smoking may affect the thyroid, inducing marked long-lasting thyroid enlargement.