Thyroid Disorders


Thyroid hormone regulates the body's metabolic activity. Because its activities are so diverse, clinical signs of thyroid disease vary enormously.

Overactive Thyroid Gland – Hyperthyroidism In Cats

Over-production of thyroid hormone is very common in cats over 10 years old and very rare in dogs of any age. Almost invariably it’s caused by a benign enlargement of the thyroid gland. An affected cat has a voracious appetite but loses weight. There is sometimes, but not always, increased drinking and urinating. There may also be increased physical activity and behaviour changes such as increased irritability and aggression. Blood samples reveal consistently elevated thyroid hormone levels. Treatment is with thyroid suppressing medication, radiation therapy to shrink the thyroid or surgical removal of the affected gland.

Underactive Thyroid Gland – Hypothyroidism In Dogs

Perhaps the most common hormonal problem in dogs (but extremely rare in cats) is underactive thyroid glands. In four out of five sufferers of 'hypothyroidism' their own immune system has attacked and destroyed their thyroids. Clinical signs are not apparent until 75 percent of thyroid tissue is destroyed. It is reported to affect one out of 250 dogs. Because the signs of disease can sometimes develop very slowly, and because other diseases cause a temporary reduction in thyroid activity, the condition is probably under diagnosed.

Signs include:

Lethargy Weaknes Oily skin (seborrhea)
Weight gain Poor hair quality Ear inflammation (otitis)
Reduced tolerance of exercise Increased skin pigmentation Skin thickening
Thinning hair Skin infection (pyoderma) Intolerance to cold
"Tragic" facial expression

Diagnosis And Treatment

Blood cholesterol and triglycerides are raised in about three quarters of hypothyroid individuals. A similar number have mild anemia. Thyroid hormone (thyroxine) levels are measured but because many illnesses and drugs such as corticosteroids temporarily reduce thyroid hormone, test results can be difficult to interpret. Most, but still not all hypothyroid dogs have reduced thyroxine levels and increased thyroid stimulating hormone (TSH) levels. A popular diagnostic test is simply to monitor the response to thyroid supplementation. This is practical but can be misleading in that even pets with normally functioning thyroid glands may grow more hair when given the standard treatment, synthetic thyroid hormone, called L-thyroxine. Ill dogs respond within days to L-thyroxine, becoming more alert and willingly taking more exercise. Weight loss is obvious within weeks but coat changes take much longer, up to 12 weeks. The prognosis is excellent. On daily L-thyroxine a dog has a normal life expectancy.

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