Thyroid disorders are common in humans but they also affect the animal world! Dr Anna
Reavell
, BTF Trustee and veterinary surgeon, gives a fascinating insight into the signs and symptoms of thyroid disorders in our pets and how they can be treated.

At ʻvet schoolʼ we were often told ʻcats are not small dogsʼ and this definitely applies to thyroid disease. I have never seen a hyperthyroid dog, and never seen a hypothyroid cat but the opposite conditions are extremely common.

In dogs hypothyroidism is usually caused by immune-mediated destruction of the gland, with less than 5% of cases caused by thyroid neoplasia, congenital disease or central nervous system disorders. Hypothyroidism is most common in middle aged dogs of either sex, with Dobermann Pinschers and Golden Retrievers having the highest incidence. As in people, dogs tend to be lethargic and gain weight. Changes in the skin are common, including a ʻtragicʼ facial expression and a hairless ʻrat tail!ʼ They may also have symmetrical alopecia along their flanks. The disease tends to creep up on dogs, often owners do not realise how affected their pets have become until they are treated.

Diagnosis is by free T4 and TSH measurement and treatment is with levothyroxine. Dogs metabolize thyroxine much faster than humans with a half life of 9-15 hours instead of 6-10 days. Consequently, a dog requires a much larger dose: a Retriever needs 800mcg daily, whereas an adult human will typically need 125 mcg per day!

Hyperthyroidism in cats was first described in 1979 and is now the most common endocrine disorder. Risk factors have not been fully established but may be linked to chronic exposure to thyroid-disrupting chemicals in the environment. The disease has been likened to multinodular goitre in humans. Thyroid stimulating immunoglobulins responsible for Gravesʼ disease in humans, have not been identified in cats.

Hyperthyroidism is seen in older cats, with the average age of onset 12-13 years, and no apparent breed or sex differences. Often owners will remark how lively and energetic their old cat has become, along with weight loss despite a very good appetite. Cats will often try to seek out cool areas and their temperament can be described as ʼspikey.ʼ There is often an obvious goitre and an increased heart rate, over 240 beats per minute (normal is about 180 bpm).

Diagnosis is by free T4 measurement and treatment usually starts medically with 5mg carbimazole twice daily. Cats are notoriously difficult to ʻtabletʼ so recently some alternative options have become available, such as a transdermal carbimazole gel to apply to the ear, or an iodine-deficient prescription diet. This, however, is not as easy as it sounds as the cat must not eat any other food or treats, or hunt wildlife, and other cats in the family must not eat this food!

Whilst medical treatment can continue for life, surgical thyroidectomy is considered gold standard. Postoperative complications of hypocalcaemia can occur if the parathyroid gland is damaged. Permanent hypothyroidism is rare so treatment with levothyroxine is not usually required. Radioactive iodine treatment is available in specialist centres, and requires the cat to be isolated for a few weeks because of the radiation risk.

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