IODINE INTAKE AND THE SEASONAL INCIDENCE OF THYROTOXICOSIS IN NEW-ZEALAND

We explored a possible relationship between the seasonal variation in the incidence of thyrotoxicosis in our area and iodine intake, as assessed by measuring the 24-h urinary iodide excretion rate eight times at 3-month intervals in a group of normal subjects and in 992 outpatient specimens over a 2-year period (1988-1990). For the period 1978-1990, 139 cases of thyrotoxicosis were diagnosed during the warmer half of the year (November-April) and 100 cases during the cooler half (May-October). This difference was statistically highly significant (P = 0.013). A similar disproportionality in the incidence of thyrotoxicosis was observed between the warmer (57%) and cooler (43%) halves of the year for the period 1988-1990. During the 1988-1990 period there was no discernible seasonal variation in either the milk iodide concentration or in the cumulative mean 24-h urinary iodide excretion rates for the normal subjects or the outpatients. We conclude that the higher proportion of thyrotoxic patients diagnosed during the warmer 6-month period of the year in our area is best explained by the fact that symptoms tend to be less tolerable in warm weather. A comparison of our findings on 24-h urinary iodide excretion rates with data collected in our area 25 years ago suggested that iodine intake may have declined.