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Thyroid Abnormality In Dogs
By Shane Jackson MA, Vet MB, MRCVS
A healthy Irish setterThis item on the thyroid gland and the effects of its malfunction on the body has been stimulated by several recent cases with which I have been involved. The interesting part from a clinician’s point of view is that many different symptoms have been displayed by these cases and this can make diagnosis quite tricky.
The thyroid gland consists of two lobes and is found lying along side of the windpipe, or trachea, just below the larynx. It produces two related hormones, imaginatively called T3 and T4, of which T3 is the active form, T4 being converted into T3 when required. However, for convenience, it is easier to refer to them both as ‘thyroid hormone’. The effect that thyroid hormone has on individual cells is deceptively simple; it increases the metabolic rate. Once active T3 has entered a target cell, various enzymes are produced which break down carbohydrate, fat and protein and this results in weight loss, increased heart rate, blood pressure and body temperature as well as having effects on the central nervous system and the gut. Thyroid hormone also has a role to play in reproduction and growth and development of the young animal.
Most cases of thyroid abnormality in the dog are because of low thyroid production, or hypothyroidism. This is unlike the cat where hyperthyroidism, or over activity of the thyroid glands, can be a major problem. However hyperthyroidism is far less common in the dog and is usually due to cancer. Canine hypothyroidism may be caused by numerous factors such as poor diet and chemical toxins including drugs, but the vast majority of cases are thought to be due to inflammation where for some reason or other the body has started to attack itself, i.e. an autoimmune thyroiditis. This is thought to be a genetic component. Any dog may be affected by hypothyroidism, but it is commoner in middle age (four to eight years) and there seems to be a predisposition for the larger breeds such as German shepherds, retrievers, and setters. The giant breed, such as Great Dane and St Bernard’s may be affected well before middle age.
As mentioned above, the clinical signs can be varied. Often the first thing noticed is poor coat quality. There may be hair loss, with concurrent slow hair growth. The coat may be dull and dry, though the skin can be greasy and thickened. Dogs often gain weight easily and become very lethargic, having a poor tolerance to exercise and to cold. The heart may be slow, bitches show irregular heat cycles and both sexes may be low on libido with poor fertility. Wound healing may be slow and there may even be various neurological changes, including seizures, in coordination and facial paralysis. To complicate matters, the main symptoms, that is weight and skin/coat changes may not be present and there may be numerous different diagnoses which can produce any or all of these symptoms. Once hypothyroidism is suspected, blood tests are helpful in confirming the diagnosis, but even here, care must be used in the interpretation of the results. Amongst other tests that may be carried out, the amount of circulating thyroid hormone can be estimated. This though, varies with age (it is higher in puppies and decreases as the dog gets older) and the levels can be affected by concurrent drug therapy and other diseases.
However, despite all this difficulty the good news is that once a case is diagnosed, treatment is easy. Thyroid supplement, in the form of tablets, is given once daily, although the treatment is usually for life. Response is quite quick; within the first week there may be some weight loss and certainly there should be a noticeable improvement in the dog’s mental state. Owners say the dog is brighter and happier. Within six week, skin problems are often vastly improved, though it may be several weeks after this that they have totally resolved. Cases where despite laboratory testing, the diagnosis is unsure, can be given a trial treatment of thyroid supplementation and treatment should be stopped if not improvement is seen within four weeks.
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