Performance and metabolic and pathological changes were evaluated in 48 growing pigs fed different levels of dietary protein, cyanide and iodine. Protein deficiency reduced urinary iodine excretion and the concentrations in serum of protein, protein bound iodine and thiocyanate. It also reduced liver rhodanese activity and caused a decrease in urinary thiocyanate excretion which was not significant. Dietary cyanide increased urinary thiocyanate and iodine excretion and serum thiocyanate concentration while the basal diet reduced urinary iodine excretion and serum PBI. Pathological studies showed that cyanide treatment had no marked effect on the microanatomy of the tissues examined. Dietary protein deficiency caused histological changes in the thyroid gland and bone which suggested a decline in metabolic activity. Iodine deficiency caused hyperplastic goitre in the experimental animals.