A five and a half-month-old, male domestic shorthair of 1.4 kg was presented with severeconstipation. Physical examination showed a dull, small cat with a poor hair coat and excessivescaling, hypothermia and a large amount of feces in the abdomen. Body proportions showeddisproportional dwarfism with a large head and a short neck and limbs. Radiographs revealedmarked epiphyseal dysgenesis with delayed maturation and ossification. Megacolon was present.Based on an undetectable level of TT4 and an elevated TSH level in serum, congenital primaryhypothyroidism was diagnosed. On scintigraphic examination, the diagnosis was confirmed.After several months of levothyroxine therapy, the cat was bright and alert, showed no signs ofconstipation and developed normally.