Hypothyroidism is the clinical condition associated with low thyroid hormone levels which causes low metabolism in most tissues. It is the most common hormonal problem in dogs.
There are two forms of hypothyroidism in dogs:
Primary acquired hypothyroidism which accounts for 90% of the cases. It can be caused by immune inflammation of the thyroid glands or idiopathic follicular atrophy resulting in thyroid dysfunction. Less commonly this form of hypothyroidism is caused by a lack of iodine in the diet and destruction of the thyroid glands by infection or tumours.
Secondary hypothyroidism is caused by a congenital malformation of the pituitary gland, or destruction of this gland by infection or tumour. This results in a lack of thyroid stimulating hormone being produced which in turn leads to low production of thyroid hormone.
What are the Signs Displayed by Affected Dogs?
Hypothyroidism occurs more commonly in females, especially speyed females, with the average age of affected dogs being 4 to 10 years of age. The most commonly affected breeds are Airedale Terrier, Boxer, Cocker Spaniel, Dachshund, Doberman, Golden Retriever, Great Dane, Irish Setter, Miniature Schnauzer, Old English Sheep Dog, Pomeranian, Poodle and Shetland Sheepdog.
The signs seen usually develop gradually and vary greatly. They include lethargy, depression, exercise intolerance, personality change, weight gain, gradual hair loss, infertility and low libido.
The following changes are often seen during physical examination:
Skin: This is the most common change seen in affected dogs. Dogs show symmetrical hair loss on both sides that starts on the tail and becomes more generalised. Often the coat becomes dull, brittle and dry, and the skin may become darker.
Heart: Some affected dogs have a decreased heart beat with a weak peripheral pulse.
Nervous system: Odd cases show local nerve deficits which include facial and vestibular (i.e. brain) nerve defects, paralysis of the larynx and Horner's syndrome. More uncommonly general nerve defects can occur causing a generalised weakness.
Reproductive system: It is not uncommon for infertility to occur in intact dogs. In the case of males there is often a low libido and shrinkage of the testicles whereas in the case of females the seasons are often shortened and the period between seasons prolonged.
Eyes: Sometimes lipid deposits occur on the cornea.
Gastrointestinal system: Some dogs show constipation or regurgitation.
How can Affected Dogs be Diagnosis?
There are a lot of different 'diseases' that present similarly to hypothyroid dogs, especially other hormonal causes of hair loss such as Cushing's disease and sex hormone responsive skin conditions. Most cases of hypothyroidism can be diagnosed by carrying out the appropriate thyroid hormone tests.
How can Hypothyroid Cases be Treated?
Affected dogs can be treated by administering hormonal supplementation (normally Thyroxine or Oroxine). The dose required to be administered varies and should be individualised to the patient. The response to therapy is gradual and a minimum of three months of treatment is recommended before effectiveness is judged. Normally affected dogs require thyroid supplementation for the rest of their life and frequent rechecks are required to evaluate the thyroid hormone levels in the body (with adjustments in the amount of supplementation given).
Overall successful management is usually very good.