Myxoedema most commonly occurs in moderate to severe cases of hypothyroidism in dogs.
Thickening of the skin occurs secondary to accumulation of glycosaminoglycans (mostly hyaluronic acid) in the dermis.
Myxoedema is most common on the forehead and face, causing a puffy appearance and thickened skin folds above the eyes. The puffiness, plus slight drooping of the upper eyelid, gives some dogs a “tragic” facial expression.
These changes also have been found in the GI tract, heart and skeletal muscles.
Myxoedema coma, a rare syndrome, is the extreme expression of severe hypothyroidism. The course can develop rapidly; lethargy progresses to stupor and coma. The common signs of hypothyroidism (eg hair loss) are present, but other signs, such as hypoventilation, hypotension, bradycardia and profound hypothermia, are usually seen as well.