This study was carried out on 25 Mongrel dogs with age ranged from 2 to 4 years, and body weight ranged between 11-23 kg. Hypothyroidism was induced in dogs by drugs or surgical methods. The drug-induced hypothyroidism (n=5) was induced by oral dosing of Sulphamethoxazole-Trimethoprime combination (~30 mg/kg. BW of sulphamethoxazole). Five dogs were dosed with saline as control. Two surgical methods were used to induce hypothyroidism. In the first method (thyroidectomy-induced hypothyroidism), both right and left thyroid glands were surgically removed (n=5). In the second method (ligation-induced hypothyroisim), hypothyroidism was induced by surgical ligation of thyroid arterial blood supply (n=5). Five dogs were used as sham control for surgical methods. The most common clinical signs recorded in the three groups were lethargy, weight gain, alopecia, and other dermatological changes. The biochemical changes included significant reduction of triiodothyroxine (TT3) and tetraiodothyroxine (TT4), increased thyroid stimulating hormone (TSH), hypercholesterolemia, hypertriglyceridemia, elevated liver enzymes (AST, ALT) and kidney function (urea and creatinine), hypercalcemia, hyperphosphatemia and hyponateremia. Ultrasonographic changes of thyroid gland in drug-induced hypothyroidism showed increase in both total and relative thyroid volume with decreased relative echogenicity. However, in ligation-induced hypothyroidism the total and relative thyroid volume and the relative echogenicity were decreased. Histopathological changes of thyroid gland (drug-induced hypothyroidism) revealed hyperplasia of glandular epithelium with papillary projection into the lumen and lymphocytic cellular infiltration, and desquamation of lining epithelium. In ligation-induced hypothyroidism, thyroid follicles were atrophied with hypertrophy of lining epithelium. These results confirmed the significant role of thyroid gland in maintaining the body metabolic equilibrium and the integrity of different organs, such as liver, kidney and skin.
KEY WORDS: Hypothyroidism, Ligation, Thyroidectomy, Total Thyroxine, Total Tri-iodothyronine, Ultrasonograph.