[Bone effects of the association hyperthyroidism-castration of adult female rats]

The relationship between thyroid, gonads and bone morphology was investigated in 5 month-old Wistar rats, castrated and kept in hyperthyroid or euthyroid state for periods of 30, 60 and 90 days. A non-castrated control group was maintained in the same condition. At the end of each period, plasma concentrations of free T4, progesterone and estradiol were measured. The bones from each group were submitted to radiological and histological analysis. Hyperthyroidism in intact rats caused variation in bone morphology throughout the experiment, leading to loss of trabecular bone tissue at day 60, due to increased bone resorption. At day 90 there was no bone loss, because the increase in bone resorption was accompanied by higher bone apposition. In castrated euthyroid rats reduction in progesterone levels inhibited the bone apposition, causing a slight loss of trabecular bone tissue in vertebrae and alveolar bone at day 30, which was intensified at days 60 and 90, affecting cortical bone tissue. The administration of thyroxine to castrated rats reversed osteopenia at days 30 and 60, which did not occur at day 90 when bone loss was more extensive. In conclusion, hypoprogesteronism and hypoestrogenism alter bone metabolism and the bone response to hyperthyroidism depends on the plasma profile of sex steroids, time of exposure and bone tissue configuration.