Effect of short-term ozone exposure on exogenous thyroxine levels in thyroidectomized and hypophysectomized rats

Short-term ozone exposure (1 ppm × 24 hr) of male rats results in a significant reduction of circulating thyroid hormones and thyroid stimulating hormone (TSH). The reduction of thyroid hormone levels after ozone exposure has been hypothesized as a possible adaptive mechanism to enhance survival of rats during ozone exposure. In this study, we investigated the effect of ozone on thyroid hormone (T₄) levels in thyroidectomized and hypophysectomized rats which received exogenous T₄ in the drinking water. Groups of normal, intact rats, thyroidectomized rats maintained on T₄ at doses ranging from 75 to 1000 μg/liter, and hypophysectomized rats maintained on 300 μg T₄/liter were exposed to ozone (1 ppm × 24 hr). Plasma T₄ concentrations were significantly reduced after ozone exposure, and the results indicated that the higher the circulating T₄ levels before exposure the more they were reduced after ozone exposure. This reduction in T₄ levels cannot be accounted for in these animals by reduced pituitary TSH levels or the effects of fasting, but is likely to be due to peripheral changes in plasma thyroid binding proteins initiated by ozone exposure.