Thyroid Hyperactivity Induced by Methimazole, Spironolactone and Phenobarbital in Marmosets (Callithrix jacchus): Histopathology, Plasma Thyroid Hormone Levels and Hepatic T₄ Metabolism

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To determine drug-induced hyperfunction of marmoset thyroids due to inhibition of synthesis or enhancement of metabolic elimination of thyroid hormones, males were orally administered 10 and 30 mg/kg/day methimazole (MMI), 30 and 100 mg/kg/day spironolactone (SPL), or 50 mg/kg/day phenobarbital (PB) for 4 weeks. MMI caused marked hypertrophy of follicular epithelial cells in accordance with a significant decrease in the plasma thyroxin (T₄) level. Hypertrophied epithelial cells were filled with dilated rough endoplasmic reticulum and reabsorbed intracellular colloids, and the luminal surface was covered with abundant microvilli. The colloid included vacuoles positive to anti T₄ immuno-staining. SPL and PB also caused similar histomorphological changes, although they were less severe than those due to MMI and were not clearly associated with decrease in the plasma T₄ levels. Hepatic T₄ UDPGT activities tended to increase due to SPL and PB treatment, however, which were not so significant as increases in microsomal cytochrome P-450 contents. Some animals treated with SPL and PB showed marked increases in thyroid weights due to inactive dilated follicles. In conclusion, hyperactivity of thyroid follicles was induced in marmosets not only due to inhibition of T₄ synthesis produced by MMI but also because of enhancement of hepatic T₄ elimination produced by SPL and PB. However, hypertrophic effects of SPL and PB were less severe than MMI, because plasma T₄ levels were maintained at almost pretreatment or control levels after SPL or PB treatment.