Dogs with hypothyroidism do not produce enough thyroid hormone, which results in a reduced metabolic rate. In most cases this is due to an autoimmune destruction of the thyroid gland which produces the hormone.
Hypothyroidism is most common in middle-aged to older dogs although any age can be affected. Certain breeds including Golden Retrievers, Dobermanns and Cocker Spaniels are predisposed to the condition. It is very rare in cats.
The symptoms of hypothyroidism can be vague, variable and non-specific. Common signs include lethargy, exercise intolerance and weight gain. These symptoms are often missed by owners, assuming they are just signs of their pet getting older. Dogs may also become less tolerant of cold, seeking out warm places to lie. Changes to the skin and coat are common such as dry flaky skin, bald areas and recurrent skin infections.
Hypothyroidism can be challenging to diagnose as the symptoms are non-specific. Thyroid hormone levels are variable and can be lowered by many non-thyroidal illnesses as well as some drugs; therefore a low level of thyroid hormone (T4) alone cannot confirm the diagnosis. Often several hormone levels (T4, TSH, fT4 and TgAB) will be measured and interpreted together to increase the accuracy of the diagnosis.
Hypothyroidism is treated by supplementing Thyroxine (T4) in tablet form. A syrup is also available. It may take 1-2 months to see improvements, particularly with skin/coat symptoms. The most important indicator of the success of therapy is the resolution of symptoms. Blood tests will also be used to check optimum Thyroxine levels and adjust the dose of medication. If symptoms persist despite medication the dose or frequency of medication may need to be changed, or the diagnosis may be incorrect.