The effects of hyperthyroidism on the fine structure of the zona fasciculata of the rat adrenal cortex

Chronic hyperthyroidism was induced in radiothyroidectomized and surgically thyroidectomized adult male rats by feeding a diet of 0.3–0.4% desiccated thyroid for four to ten week periods. Short term hyperthyroidism was induced in intact rats by the intraperitoneal injection of 250 μg/day of sodium levothyroxine for ten days. Adrenal glands were fixed by perfusion with 3% phosphate buffered glutaraldehyde and post‐fixed in 1% osmium tetroxide. The zona fasciculata of the adrenal glands from experimental and control animals was examined in the electron microscope. Drastic mitochondrial alterations characterized the zona fasciculata of hyperthyroid rats. Initially, mitochondria became more nearly round and contained increased numbers of paracrystalline tubular inclusions of the type described by Kjaerheim ('67) and Wheatley ('68). In more altered states many mitochondria were filled with clusters of parallel tubular cristae of varying complexity which at times enveloped islands of mitochondrial matrix and fingers of cytoplasm which projected into the mitochondrion. Aggregations of numerous mitochondria occupied much of the cytoplasm of highly altered cells. Modifications of this nature have not been described following ACTH stimulation of the adrenal cortex. This raises the distinct possibility that thyroid effects on the adrenal cortex are not entirely ACTH mediated.