RESUMO
The effect of in vivo administration of cadmium chloride on the pituitary-thyroidal axis was assessed in 200 g body weight Wistar rats. A dose of 2.5 mg/kg body weight was injected i.v. 24 h before the experiments were initiated. Plasma thyroxine (T4) and tri-iodothyronine (T3) concentrations in cadmium-treated rats were significantly (P < 0.01) decreased, whereas plasma TSH failed to increase in response to low T4 and T3. However, the TSH response to TRH and the pituitary content of TSH in these rats were both normal. Cadmium induced a significant (P < 0.01) decrease in 4-h thyroidal 131I uptake and in thyroid/plasma radioactivity ratio. The in vitro conversion of T4 to T3 in the pituitary was significantly (P < 0.01) blocked by cadmium whereas there was no in vivo effect. Parameters of peripheral T4 kinetics in cadmium-treated rats, such as metabolic clearance rate (P < 0.01), fractional turnover rate (P < 0.01), absolute disposal rate (P < 0.05), urinary clearance (P < 0.05) and faecal clearance (P < 0.05), were all decreased by cadmium. The lack of response of TSH to low plasma T4 and T3 and the normal response to exogenous TRH in this and in other non-thyroidal illness syndromes produced by other pathologies suggest a decreased stimulation of pituitary thyrotrophs by endogenous TRH.
