Hypothyroidism, also called underactive thyroid, is an endocrine disorder in which the thyroid gland does not produce enough thyroid hormone. It is relatively uncommon in dogs and extremely rare in cats.
Canine hypothyroidism is seen in middle-aged dogs, generally around the age of seven. It occurs most often in medium-sized or large dogs, but can affect any breed. Some breeds are more predisposed, including Airedale Terrier, Beagle, British Bulldog, Poodle, Chow Chow, Dobermann, Golden Retriever, Labrador, Pomeranian, Irish Setter, Shar-Pei, Shetland Sheepdog (Sheltie) and Dachshund.
The thyroid gland regulates metabolism. An underactive thyroid slows down all of the body’s processes. Hypothyroidism has several underlying mechanisms. For example, the thyroid may be unable to produce sufficient quantities of thyroid hormone T4. In some cases, the pituitary gland does not secrete enough thyroid-stimulating hormone (TSH), affecting the thyroid; in other cases, the hypothalamus does not secrete enough thyroid-releasing hormone (TRH) to stimulate the pituitary, producing a cascade effect.
The clinical symptoms of canine hypothyroidism are non-specific and vary from one animal to the next. In general, weight gain (without any change in appetite), lethargy and exercise intolerance are commonly observed. The coat is dry, dull and sparse, the skin scaly, hyperpigmented, oily and more prone to infection. More severely affected dogs may tire easily and have hind leg weakness, facial nerve paralysis, hypothermia (abnormally low body temperature) and breathing difficulties, sometimes accompanied by voice changes. They have a slower heart rate and their reproductive system shuts down. Males have decreased libido and females have absent or irregular heats. Fatty deposits within the cornea (corneal lipidosis) or decreased tear production leading to dry eye syndrome (keratoconjunctivitis sicca) can result in sight loss.
Canine hypothyroidism can also be congenital (present from birth). In this case, it is associated with disproportionate dwarfism, bone abnormalities and sometimes a goitre, in addition to the other clinical symptoms mentioned.
Given the varied clinical picture, additional examinations are required to make a diagnosis.
Thyroid biopsy is the best diagnostic method, but is difficult to perform and rarely used in practice.
More common in veterinary practice is a blood test, which usually shows non-regenerative anaemia in one-third of hypothyroid dogs, while more than two-thirds have abnormally high cholesterol and triglyceride concentrations, associated with a decrease in sodium levels. Liver enzymes are also commonly elevated.
More specifically, a blood test can also measure thyroid hormones (total T4, free T4 and TSH). Interpreting the results may be difficult, however, as they may be distorted by other (intercurrent) conditions or by medication. If the test results are conclusively positive, treatment of hypothyroidism is initiated. If the results are inconclusive, the blood test is repeated after treating the other conditions. Medical imaging techniques, such as ultrasound and scintigraphy (gamma scan), are sometimes used.
Treatment consists of oral supplementation of thyroid hormones, ideally in the fasted animal. Dosage should be patient-specific. Dogs with concomitant heart disease need special attention, as a sudden increase in thyroid hormone levels influences cardiac function.
It is not always easy to find the right dosage of thyroid hormones. Treatment effectiveness is assessed on the basis of two criteria: improvement in overall condition and blood parameters restored to their normal range. General clinical symptoms such as lethargy and exercise intolerance are the first to disappear following treatment. It takes at least four to eight weeks to determine whether treatment has been effective, but it may be several months before the dermatological symptoms improve.
In conclusion, the treatment of canine hypothyroidism is relatively simple and effective.
Dr Isabelle de Grand Ry
DMV | GPCert In Small Animals Surgery
Vet Center l’ETOIL