Affects: Dogs
Hypothyroidism is an endocrine disorder in which the thyroid gland fails to produce sufficient thyroid hormones. Hypothyroidism is one of the most common endocrinopathies in dogs. It is either acquired or congenital.
Signs And Symptoms: Signs of hypothyroidism vary based on factors such as the type and breed. Due to the slow onset and progression of the disease, clinical signs do not initially appear; for example, they do not appear until the 3rd stage of lymphocytic thyroiditis. It is not uncommon for owners to ignore certain signs of hypothyroidism due to the slow progression of metabolic symptoms. Often, metabolic symptoms will go unnoticed until after a dog has received treatment for hypothyroidism.
The most common signs of hypothyroidism are dermatological and changes related to decreased metabolism. In one study, the most common symptoms were dermatological conditions, obesity, and lethargy. Nearly 9/10 dogs had dermatological symptoms, and lethargy and obesity were observed in almost half of all dogs.
A lowered concentration of thyroid hormone affects organs throughout the body.
Causes: The causes of primary hypothyroidism include:
Lymphocytic thyroiditis
Idiopathic atrophy
Neoplasia destroying the thyroid gland
Diagnosis: Given the possible comorbidities and other causes of low serum concentrations of thyroxine, it can be difficult to definitively diagnose hypothyroidism. A strong cause for a diagnosis exists when clinical signs are showing, test results are indicative, and no other illness is present. To exclude the possibility of a thyroid-responsive disease, thyroid supplementation treatment can be ceased once symptoms have disappeared. If symptoms recur after 6–8 weeks, this excludes a thyroid-responsive disease.
Treatment: The main form of treatment for all forms of hypothyroidism is levothyroxine sodium. This method allows for normal levels of thyroid hormone in tissue and elsewhere whilst keeping the risk of iatrogenic hyperthyroidism low. The treatment dose for dogs is 10 times that of humans due to a shorter half-life of serum thyroxine and poorer gastrointestinal absorption. Many factors, such as whether treatment is once daily or twice daily, the route of administration, and the animal's physiology, affect treatment, and patients need to be monitored closely whilst undergoing treatment. It is not recommended to change a patient's formulation. Serum concentrations of thyroxine and thyroid-stimulating hormone should be measured every 6–8 weeks, or more frequently if the animal does not respond well to treatment or thyrotoxicosis occurs. Measurements should be taken 4–6 hours after levothyroxine treatment in dogs receiving twice-daily treatment and 4–6 hours before and after in dogs receiving once-daily treatment. In severe cases, such as myxoedema coma, when hypometabolism is occurring, the treatment should be administered intravenously.
Evaluation of treatment should not occur until at least 6–8 weeks have passed. Symptoms of hypothyroidism should resolve if treatment is effective. Lethargy and torpidity are usually resolved within the first month, and obesity by around two months; however, other factors can impact obesity. While symptoms should have resolved within 6–8 weeks, full regrowth of the coat following endocrine alopecia may not occur for several months after starting treatment. Myocardial improvement can occur as early as a month, but in some patients it will not occur until around a year has passed.
Prognosis: A dog with treated primary hypothyroidism should have a life expectancy equivalent to that of clinically healthy dogs. Prognosis is worse for myxoedema coma and congenital hypothyroidism. Death is still likely following early treatment for myxoedema coma. Prognosis is poor even for treated dogs with congenital hypothyroidism; many of the effects, like cretinism and retarded growth, result in lifelong complications such as osteoarthritis. The prognosis for secondary hypothyroidism is poor due to the tumour and pituitary gland destruction.