Hypothyroidism is a hormonal condition in which dogs do not produce enough thyroid hormone. This leads to various symptoms including lethargy and weight gain. Hypothyroidism is normally treated with ongoing medication.
What does the thyroid gland do?
The thyroid gland is located in the neck, by your dog’s voice box. There are two lobes of the gland which are found either side of the windpipe.
Animation on the anatomy of the thyroid gland:
The thyroid gland's main function is to produce thyroid hormone. Production of this hormone is controlled by two areas of the brain, the hypothalamus and the pituitary gland (hypophysis).
These brain areas stimulate the thyroid gland to produce thyroid hormone. The hormone in the bloodstream then inhibits the signals from the hypothalamus, making a 'negative feedback' loop to ensure that the body does not produce too much.
Animation on the production of thyroid hormone:
Most cases of hypothyroidism in dogs are classed as 'primary hypothyroidism', in which the cells of the thyroid gland are destroyed. This may be due to an over-reaction of the immune system. It is also possible for dogs to develop 'secondary hypothyroidism', which is slightly different as it begins with a problem in the pituitary gland. The lack of stimulation from the pituitary gland then causes the thyroid gland to shrink. Primary and secondary hypothyroidism are shown in the following animations.
Animation illustrating primary hypothyroidism:
Animation illustrating secondary hypothyroidism:
What happens in hypothyroidism?
Thyroid hormone basically sets the rate of the dog's metabolism. When the thyroid gland is underactive, this means your dog's metabolism will be slowed down. This causes a range of signs:
- Weight gain despite no increase in food intake
- Slowing of the heart rate
- Hair loss -usually symmetrical on the flanks with no associated itchiness. The skin may also appear thicker and darker
- Decrease in sexual behaviour
- Lethargy and mental dullness, and reluctance to exercise
- Dogs may also become sensitive to the cold, and seek out heat sources
In rare cases, when hypothyroidism is very severe, dogs may fall into a coma.
Animation on clinical signs associated with hypothyroidism:
The condition normally develops between the ages of about 4 and 10 years. As the disease often develops in later life, the signs may be mistaken for normal ageing.
Which dogs are more likely to develop hypothyroidism?
Hypothyroidism is most common in middle-sized to large breed dogs. Predisposed breeds include the Golden Retriever, Miniature Schnauzer, Dobermann Pinscher and Irish Setter. Some smaller breeds like Dachshunds are also predisposed.
How will my vet diagnose hypothyroidism?
Your vet may suspect that your dog has hypothyroidism after examining them and talking through their signs with you. However, more tests will be needed to diagnose the condition. Your vet may suggest blood tests to assess the level of thyroid hormone as a first step. Thyroid hormone can sometimes be low if dogs are ill for another reason though, so your vet may advise other investigations. These may include tests to assess the hormones produced by the brain to stimulate the thyroid gland, as well as blood tests to assess your dog's general health.
How can hypothyroidism be treated?
Hypothyroidism is usually treated by giving thyroid hormone tablets to make up for the fact that your dog is not producing enough of the hormone themselves. Your vet will help you work out the best dose for your dog. After treatment is started, they may suggest adjusting the dose depending on how your dog is doing. Blood tests for thyroid hormone levels will help monitor your dog's progress.
Treatment is usually lifelong, but in most cases it is effective at controlling the symptoms and improving quality of life.
Copyright & Animations ©Medi-Productions 2021