Seasonal variation in thyroid size in healthy males.

Thyroid volume, measured ultrasonically, and serum levels of T4, T3 and TSH were determined every season during one year in 13 healthy males. A mean variation in thyroid volume of approximately 23% between minimum values (15.8 +/- 1.7 ml, summer) and maximum values (19.5 +/- 1.9 ml, winter) was found (P less than 0.01), although no significant differences in the other thyroid variables could be demonstrated. This seasonal variation in thyroid size should be taken into account when goitre frequency, goitrogenic action of drugs, and goitre treatment effects are evaluated.