Thyrotoxicity of the chlorides of cadmium and mercury in rabbit

Exposure to heavy metals such as cadmium and mercury is of immediate environmental concern. The present study was aimed at establishing a direct relationship between heavy metal poisoning and thyroid dysfunction. Cadmium and mercury treatment at LD50 levels resulted in severe thyrotoxicosis in the rabbit. Within 24 h of intramuscular administration of cadmium chloride 15 mg.kg-1 body weight (bw) and mercury chloride 20 mg.kg-1 bw, thyroid peroxidase activity increased significantly over the control with a concomitant rise in the triiodothyronine (T3) titre. On the other hand, there was a remarkable fall in the thyroxine (T4) level, and the T3/T4 ratio was high as compared with the control. Evidence indicates that acute heavy metal lethality will induce immediate hyperthyroidism. It is suggested that T3-toxicosis may be produced by a preferential synthesis of T3 and/or preferential deiodination of T4 to T3. Measurement of T3 and T4 levels may thus be utilized as a reliable indicator of heavy metal lethality.