Hypothyroidism is due to impaired production and secretion of the thyroid hormones, which results in a decreased metabolic rate. It is the most common endocrinopathy of the dog. The clinical signs are quite variable and almost any organ system may be involved.
Causes of hypothyroidism
The thyroid gland is a small gland that is situated close to the larynx (voice box) in the neck. It is regulated by the small pituitary gland that is located at the base of the brain which produces a hormone called thyroid stimulating hormone (TSH). This hormone stimulates the thyroid gland to produce thyroxine, the thyroid hormone. The pituitary gland responds to the blood level of thyroid hormone by producing more TSH if the thyroid hormone level is low and less TSH if the thyroid hormone level is high. The disorder results as primary, secondary, tertiary and congenital forms and of which, primary hypothyroidism accounts for almost 95 percent of the clinical cases.
Who gets affected?
This condition is commonly noticed in middle-aged dogs between 4 to 10 years of age and affects mid to large size breeds. Golden Retriever, Doberman Pinscher, Irish Setter, Miniature Schnauzer, Dachshund and Cocker Spaniel are the common breeds at risk. There does not appear to be a sex predilection but spayed females appear to develop it more often than intact females.
The clinical signs of hypothyroidism may be vague and insidious in onset, therefore hypothyroidism may be considered in the differential diagnosis of a wide range of medical problems. There is not a specific symptom that is diagnostic for hypothyroidism. The classical dermatological manifestations occur in 60-70 percent of hypothyroid dogs. These may include a dry hair coat, seborrhoea, alopecia, hyperpigmentation and pyoderma. While hair loss occurs in a bilaterally symmetrical pattern, it initially occurs in areas of friction such as the tail, around the neck, lateral trunk and ventral thorax. Lethargy, mental dullness, weight gain, unwillingness to exercise and cold intolerance are the signs that result from decreased metabolic rate. Accumulation of excessive amounts of glycosaminoglycans (mostly hyaluronic acid) in the dermis results in the myxoedematous appearance (tragic facial expression) found in some dogs. Neurological, cardiovascular (bradycardia) and reproductive manifestations have also been recognised. Myxedema coma, a rare syndrome, is the extreme expression of severe hypothyroidism.
Unusual signs include seizures, neuromuscular disorders and peripheral neuropathies. These manifestations are related to the profound hyperlipidemia. Two specific diseases associated with hypothyroidism are megaesophagus and laryngeal paralysis. A loss of smell and taste are also possible. The cornea might undergo fat (lipid) deposits or become ulcerated. Changes with adequate tear production along with internal structures of the eye could occur. Abnormalities in heart strength, rate and rhythm, along with atherosclerosis, could also occur. Inadequate thyroxine makes the immune system less effective at fighting infections, especially the bacterial skin infections (pyoderma) that occur secondarily. Suppression of the immune system might even increase susceptibility to Demodicosis and Malasseziosis. Breeding dogs might have abnormal heat cycles, infertility and high puppy mortality. In addition to low thyroxine, hypothyroidism is implicated in diabetes mellitus and Addison’s Disease and musculo-skeletal disorders.
A study on hypothyroid dogs revealed the following variety and frequency of signs seen with the disease:
Hypothyroidism in dogs is probably one of the most over diagnosed diseases in small animal practice. The clinical signs of many diseases mimic those of hypothyroidism. There is no single hamatologic or biochemical test that is conclusive for hypothyroidism and even hormonal tests must be interpreted in light of historical and physical findings. The wide variety of clinical signs and findings associated with hypothyroidism necessitates specific testing of thyroid function to establish a definitive diagnosis. Tests currently available for diagnosing thyroid disease include total thyroxine (TT4), total tri-idothyronine (TT3), free thyroxine (fT4), thyroid stimulating hormone (TSH), TSH response test, TRH response test, T4 and T3 autoantibodies, antithyroglobulin antibodies, nuclear scintigraphy and thyroid gland biopsy. The test chosen will depend on the symptoms and the availability of different tests.
Total T4 Test: This is the most common preliminary test for hypothyroid patients. The TT4 hormone is produced only in the thyroid gland and dogs with a failure of the thyroid gland will have a lowered level of this hormone. However, there are other conditions that can cause a lowering of TT4. So, if this screening test is positive for hypothyroidism, another more specific test is often done to confirm the diagnosis.
Free T4: T4 is present in two forms in the body. The “bound” form is attached to proteins in the blood and is unable to enter the cells. The “free” T4 is not attached to proteins and can readily enter the cells and perform its function. The free T4 is normally present in very small amounts.
TSH Level: In a hypothyroid dog, the TSH level will be elevated because the body is trying to stimulate the thyroid gland to produce more thyroid hormone. If the total T4 and Free T4 are low and the TSH is elevated, a diagnosis of hypothyroidism can be made.
TSH Response Test: The TSH response test has long been recognized as an accurate measure of thyroid function and serves as the “gold standard” measurement in many studies evaluating thyroid function tests. This is because it provides important information about thyroid secretary reserve. Measurement of TT4 before and six hours after intravenous administration of 0.1 U/kg bovine TSH is the recommended protocol. Post-TSH TT4 concentrations above 30 mmol/L are normal, while TT4 less than 20 mmol/L is diagnostic for hypothyroidism.
Thyrotropin Releasing Hormone (TRH) Response Test: The TRH response test is used in humans to differentiate primary from secondary hypothyroidism. In dogs, the test has been used in place of the TSH response test and change in TT4 has usually been measured.
Therapeutic Trial: When diagnostic tests do not provide a clear diagnosis of hypothyroidism, thyroid replacement therapy is suggested as a valid diagnostic step. A positive response to therapy should be interpreted with caution because clinical signs may also improve in euthyroid animals treated with L-thyroxine. A diagnosis of hypothyroidism based on response to therapy should be confirmed by recurrence of clinical signs after withdrawal of supplementation.
Treatment of hypothyroidism
Hypothyroidism in dogs is easily treated. Treatment consists of placing the dog on a daily dose of a synthetic thyroid hormone called levothyroxine. The dose and frequency of administration of this drug varies depending on the severity of the disease and the individual response of the animal to the drug. With few exceptions, replacement therapy is necessary for the remainder of dog’s life. Treatment should be initiated at a dose of 0.02 mg/kg orally every 12 hours and then the dose should be adjusted based on results of therapeutic monitoring. Using twice-daily treatment initially improves the likelihood of response to treatment in all dogs. After clinical signs resolve and TT4 concentrations stabilize within the therapeutic range, the majority of dogs can be maintained on 0.02 mg/kg once daily. The most important indicator of the success of therapy is clinical improvement. Clinical resolution of metabolic signs such as lethargy and mental dullness can be expected within two weeks of starting therapy, while other abnormalities, including dermatologic signs, may take up to three months to resolve.
(Dr K Satish Kumar works at Department of Veterinary Clinical Medicine, College of Veterinary Science, Hyderabad while Dr D Srikala is from College of Veterinary Science, Tirupati.)