Secondary iodine deficiency was experimentally produced in growing male lambs by oral administration of 50 mg/kg bodyweight of Thiourea daily for 3.5 months. At the end of the experiment the animals became weak, emaciated, anaemic, significantly reduced in body weight with facial oedema and alopecia at thigh, legs and abdomen. The clinical analysis showed significant reduction in erythrocyte and leucocyte numbers and in levels of triiodothyronine and testosterone at the end of the experiment. The histopathological picture of the thyroid gland revealed hyperplasia of the follicle-lining epithelial cells which project into the lumen. The lumens of the follicles are devoid of colloid. The testes showed ill-developed small, empty seminiferous tubulcs. In the liver, the hepatocytes showed degeneration and vacuolation with proliferation of Kupffer cells, which contain haemosiderin pigment. The kidney showed glomerular lipidosis with accumulation of haemosiderin pigment in the cytoplasm of the renal tubules. Hyperkeratosis of the epidermis associated with excessive keratin formation within the hair follicles was detected. In conclusion, deficiency of iodine causes hypothyroidism which leads to retardation of growth, reduced wool production and interferes with sexual maturity of growing male lambs.