Chronic iodine deprivation attenuates stress-induced and diurnal variationin corticosterone secretion in female Wistar rats

Many millions of people throughout the world are at risk of developing iodi ne deficiency-associated disorders. The underlying effects of iodine defici ency on neuroendocrine function are poorly defined. We have studied stress- induced and diurnal variation in corticosterone secretion in female rats re ndered chronically hypothyroid by feeding them an iodine-free diet for 6 mo nths. Corticosterone secretory responses in iodine deficient animals were c ompared to those seen in animals rendered hypothyroid with propylthiouracil and untreated controls. By using a well-validated, automated blood samplin g system to collect small samples of blood over the complete daily cycle in unrestrained animals, we have demonstrated for the first time that the nor mal diurnal rhythm of corticosterone secretion is lost in chronic iodine de ficiency and that the corticosterone secretory response to the psychologica l stress of 10 min exposure to white noise is attenuated. Despite restorati on of circulating triiodothyronine and thyrotropin releasing hormone- and t hyroid stimulating hormone beta -transcript prevalence in the hypothalamus and pituitary, respectively, 1 month after restoration of normal iodine-con taining diet both the diurnal Variation in corticosterone levels and the co rticosterone secretory response to the noise stress remained reduced in amp litude compared to control animals. Thus, chronic hypothyroidism induced by iodine deficiency significantly attenuates hypothalamopituitary-adrenal ax is activity, an effect that persists after functional recovery of the thyro id axis.