Seasonal Variation in Thyroid Function

Seasonal variation in the incidence of non-toxic endemic goitre occurring in children has been reported in southern Tasmania. Similar seasonal variation has been noticed in the increased incidence, reported recently, of thyrotoxicosis occurring throughout Tasmania. The present study shows a seasonal variation within the normal range of protein-bound iodine levels in euthyroid people. A rhythm of thyroid function may occur throughout the year, with an increase in activity in spring and early summer and a decrease in autumn and early winter. Seasonal variation in temperature and hours of daylight may be the cause of these three phenomena. It seems unlikely that any goitrogenic substance is Involved.