Effect of cyclophosphamide on thyroid gland in mice and guinea pig

Male BALB-/c mice and guinea pigs were given cyclophosphamide (CY)-either a single dose of 20 or 200 mg kg-1 or multiple doses of 2 or 20 mg kg-1 daily for 10 days. On the 1st, 3rd, 7th and 15th day after the last dose the serum levels of TSH, T4, T3 and rT3 were measured by RIA. The thyroids were processed for light and transmission electron microscopy. Both acute and chronic administration of CY resulted in a significant and prolonged decrease of serum T4 levels in mice and guinea pigs. In contrast, significant increase of rT3 level was found after long-term administration of 20 mg CY kg-1 and single administration of 200 mg CY kg-1 to mice or guinea pigs, while no changes in T3 and TSH levels were found. The thyroid 131I organifications was inhibited. In follicular cells mitochondrial damage, distention and vacuolization of the endoplasmic reticulum cisternae were established. A period of 15 days was neither long enough for serum T4 to return to the normal level nor for the morphological characteristics of the thyroid to be restored.