Pigmentation and dysfunction of Gunn rat thyroid: correlation between morphological and biochemical data

The thyroid gland of homozygous Gunn rats is moderately enlarged and displays a brownish-black discoloration. Light microscopic examination discloses that the follicular cells are filled with brown granules, which are shown, under the electron microscope, to be modified colloid droplets. Most of them possess a strong acid phosphatase and a mild peroxidase activity and contain a melanin-like pigment, according to histochemical analysis. In comparison with normal Wistar rats, Gunn rats possess significantly higher plasma thyroxine and lower triiodothyronine as well as an increased plasma TSH level. The soluble protein content of the thyroid is reduced in the Gunn rat, as is the total intrathyroid iodine content. The hyperthyroxinaemia of homozygous Gunn rats is due to a hereditary deficiency in hepatic glucuronyl transferase activity. The excess circulating thyroxine is of little functional importance because it is firmly bound to plasma proteins. But Gunn rats have a slight hypothyroid goitre for reasons not yet elucidated. The functional as well as morphological data at present available suggest a modified thyroid iodine metabolism and an altered composition of the thyroglobulin which may induce abnormalities in colloid proteolysis. The observed pigment may result from peroxidation of tyrosine. These alterations are probably independent of the sole enzymatic deficiency so far encountered in these animals and may probably be ascribed to a primary enzymatic defect in the thyroid gland itself.