Hypothyreoidism

Diminished thyroid gland function or hypothyreoidism

Hypothyreoidism is an illness caused by decreased secretion of the thyroid gland hormones T4 (thyroxine) and T3 (tri-iodothyronine). Hypothyreoidism is a common disorder in dogs but extremely rare in cats. The condition is more common in middle-aged to elderly, large-breed dogs, with some breeds being affected more than others (Giant Schnauzers, Dobermanns, Golden Retrievers, Great Danes, ....)

Symptoms

The nickname ”great pretender” is sometimes used to describe the symptoms of hypothyreoidism. The hormones secreted from the thyroid gland regulate the functioning of many different organs and systems in the body and therefore there is a wide variety of different symptoms associated with this condition. Among the most typical are general lethargy and apathy, weight gain, symmetrical loss of hairs, an overly dry or greasy coat and being cold. In addition the dog may have either diarrhea or constipation, a decreased heart rate, infertility, anaemia, recurrent skin inflammations, eye diseases, neurological issues etc.

Causes of hypothyreoidism

In approximately 95% of the affected dogs the condition is due to the destruction of the thyroid gland tissues (primary hypothyreoidism). The clinical signs manifest only when some 75% of the tissue has already been destroyed. In these cases there is usually a lymphocytic thyreoiditis or an idiopathic atrophy of the thyroid gland, or very rarely the destruction may be due to a tumor. Lymphocytic thyreoiditis (inflammation of the thyroid gland) is an immune-mediated disease and it is suspected to be hereditary.

In secondary hypothyreoidism the reason for the decreased thyroid hormone secretion is decreased thyrotropin secretion from the hypophysis in the brain. Thyrotropin (or thyroid stimulating hormone, TSH) is a hormone that regulates the functioning of the thyroid gland and its levels can decrease from either malformations or tumors of the hypophysis. Secondary hypothyreoidism is seen rarely.

There exists also a third form of hypothyreoidism, tertiary hypothyreoidism, but this form has never been accurately identified in dogs.

Inborn hypothyreoidism (cretinism) is rarely encountered, the affected puppies fail to grow and develop normally. This condition is hereditary in Miniature Fox Terriers and a DNA-test exists for testing against this condition.

Diagnosis

Diagnosing hypothyreoidism is difficult. There are several tests to diagnose the condition but unfortunately none of the tests is perfect.

To verify the condition the levels of the thyroid hormones (total T3 and T4) and TSH can be measured. The T4-levels in the body change drastically during the day and even healthy dogs may at some point have T4 values below reference levels. The level is also subjec to a variety of factors such as age and breed: small breed dogs usually have higher levels of T4 than large-breed dogs, young dogs have higher levels than older dogs, and for example some sighthound breeds such as Greyhounds and Scottish Deerhounds have lower levels than other breeds. The T4-level rises in bitches during diestrus and pregnancy. In many diseases the T4-level can be low even without hypothyreoidism (sick euthyroid syndrome), as can happen for example in Cushing's disease, diabetes, Addison's disease, during liver, kidney or heart failure or infections. Many medicaments also affect T4-levels, for example antiepileptics (phenobarbital), cortison, sulpha, clomipramin and furosemide are such drugs.

For the above reasons measuring the T4-level is not a sufficient test to detect hypothyreoidism. Usually a T4-level that is within the reference range can rule out hypothyreoidism,  but a T4-level below the reference range does not automatically imply hypothyreoidism.

The T3-level fluctuates even more than the T4 and therefore it is not usually measured to detect hypothyreoidism in dogs.

Blood samples from dogs with hypothyreoidism frequently show mild anaemia, elevated cholesterol and triglyseride levels and occasionally also elevated liver enzyme levels (ALAT and AFOS).

It is also possible to measure the amount of unbound or metabolically active T4 (vT4) in the blood. The measurement, done using a dialysis method, is a more precise and sensitive test than basic T4 and better reflects the thyroid glands activity.

A higher than normal TSH level incidates a diminished thyroid gland output. However the TSH measurement alone is not sufficient to verify or rule out hypothyreoidism conclusively, since 13-38 % of dogs with hypothyreoidism have TSH values within the reference range and 7-18% of healthy dogs have elevated values.

At the moment the diagnosis is based on the joint measurement of both T4 and TSH: a low T4, an elevated TSH and a T4/TSH ratio lower than 12 are enough to verify hypothyreoidism.

In order to detect thyreoiditis the level of thyroglobulin antibodies (TGA) in the blood can be measured. Approximately half of the dogs with hypothyreoidism have these antibodies. A positive result indicates thyreoiditis but does not imply impaired thyroid gland function. In an asymptomatic dog the result is considered merely as a sign of an increased risk of developing hypothyreoidism. An asymptomatic thyreoiditis can last for a long time before triggering hypothyreoidism, and at the moment there is some doubt whether TGA-positivity even necessarily always leads to a clinical hypothyreoidism. Vaccinations have been noticed to temporarily cause the antibody titres to rise.

To diagnose thyreoiditis T3 and T4 antibodies can also be used but they are rarer than TGAs. The T3, T4 and thyroglobulin antibodies can be used as an additional diagnostic tool to supplement the other tests.

Treatment of the condition

To treat hypothyreoidism synthetic L-thyroxine (T4) is used orally twice per day. The dosage for dogs is significantly higher than what is used for humans.The treatment is life-long but the prognosis is very good: the affected dogs can lead a perfectly normal life and the disease typically causes no shortening of the lifespan. After the medication has been started, the activity level of the dog usually resumes to normal in 1-2 weeks and the coat is regrown in a few months.

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