Changes in thyroid function after short-term ozone exposure in rats.

Exposure of male rats to ozone for 24 hrs at 1 ppm caused a profound depression of the pituitary-thyroid axis as indicated by a highly significant reduction of circulating thyrotropin hormone (TSH), thyroid hormones (T4 and T3), and protein-bound iodine (PBI). The metabolic clearance of TSH was not altered during ozone exposure and the high TSH levels seen in thyroidectomized rats were also not affected. Circulating prolactin (PRL) levels were significantly elevated after exposure. Pituitary TSH and PRL content was considerably increased in ozone-exposed rats; however, only TSH was released significantly above control values in vitro. Thyroid weight was also significantly increased after exposure. The results suggest that the depression of the pituitary-thyroid axis may be an adaptive mechanism during ozone exposure by reducing hypothalamic stimulation via thyrotropin releasing hormone (TRH) and at the same time lifting the hypothalamic catecholamine inhibition on PRL release. Both may be necessary alterations in order to develop tolerance during ozone exposure.