Clinical Signs
Clinical sings of hypothyroidism develop in dogs following destruction of greater than 75 percent of the thyroid gland. The most common signs of hypothyroidism include weight gain, lethargy, reduced activity, loss or thinning of the fur, dull hair coat, excess shedding or scaling.
Hair loss (alopecia) is seen primarily along the trunk, sparing the head and legs. The hair loss is usually not accompanied by pruritius or erythema of the skin. The hair coat tends to be thin and dull, and epilates easily. Hyperpigmentation and lichenification can also been seen and usually occur with long-standing disease. Seborrhea, inflammation and pruritius due to secondary bacterial or fungal infections may also be seen. Hypothyroid dogs often have ear infections and show ear pain, redness and odor.
Neuromuscular signs caused by peripheral neuropathies, including muscle wasting on the head, facial paralysis, head tilt, disorientation, muscle weakness or paralysis, and lameness, can be seen in dogs with hypothyroidism. Central nervous system signs (seizures, coma, ataxia, circling) are uncommon.
Reproductive abnormalities (abnormal estrus cycles, infertility, high puppy mortality. testicular atrophy and low sperm or no sperm) can be seen in hypothyroid dogs used for breeding. Less common clinical signs include cardiovascular abnormalities (bradycardia, electrocardiographic and echocardiographic changes), corneal lipid deposits and myxedema coma.
Hypothyroidism can also cause insulin resistance in diabetic dogs, resulting in poorly controlled diabetes.