The clinical signs of hypothyroidism are not specific and may resemble many other diseases

Also, the diagnostic tests are not 100% accurate and are often affected by drugs, hormones, obesity, other skin diseases and systemic diseases

A definitive diagnosis relies on a combination of history, appropriate clinical signs, results of a routine data base, basal serum T3/T4 and endogenous TSH levels

The treatment of hypothyroidism, regardless of its cause, is thyroid hormone supplementation.

What is hypothyroidism?

• Hypothyroidism is a condition in which the thyroid glands are not functioning normally.  
• Hypothyroidism is a deficiency of triiodothyronine (T3) and thyroxine (T4) which are thyroid hormones, that results from inadequate formation or secretion or improper peripheral cell interactions with T3 and T4.
• Naturally occurring hypothyroidism is common in dogs but rare in cats.

Anatomy and physiology

• Two thyroid glands are located in the neck of dogs and cats.
• The thyroid glands are regulated by the brain to release thyroid hormone.
• The function of the thyroid gland is to control the metabolic rate of cells within the body - it basically is like the throddle of an engine...which controls how fast the engine runs and how much fuel is burned.
• The lack of thyroid hormone slows the metabolic rate of the body.

Clinical signs

• Signalment
  • Hypothyroidism effects almost exclusively dogs; both small and large breed dogs develop the condition.
  • Occasionally cats can develop hypothyroidism following radioactive Iodine therapy or removal of the thyroid glands.
  • Males and females are affected equally and usually occurs between the ages of 2 to 6 years.
• The clinical signs of hypothyroidism are not specific and may resemble many other diseases.
• No one sign is specific of this condition but often a number of clinical signs seen together will prompt a veterinarian to suspect hypothyroidism.
• General signs related to a slowing of cellular metabolism include: lethargy behavior such as a lack of interest in play and frequent napping. The most common sign of hypothyroidism is increase in body weight, with almost half of the affected dogs demonstrating this sign.
• Dermatologic (skin) disease (example: poor thin coat, hair loss especially on the trunk or tail, discoloration or thickening of the skin, increased skin infections and increased scratching) are seen in about one third of the cases.
• Reproductive problems such as failure of females to go into season, prolonged estrual bleeding and male infertility are also common.
• Cardiac (heart) signs including bradycardia (slow heart rate), ocular signs and gastrointestinal symptoms are occasionally seen.
• Some of the affected dogs show weakness and a decrease in exercise tolerance, while 4% have grand mal seizures.

Diagnosis

• Hypothyroidism is often called the great imitator, because it so frequently resembles other disease conditions. Also, the diagnostic tests are not 100 % accurate and are often affected by drugs, hormones, obesity, other skin diseases and systemic diseases. A definitive diagnosis relies on a combination of history, appropriate clinical signs, results of a routine data base, basal serum T3/T4 and a TSH stimulation test.
• Thyroid function tests:
  • Baseline T4 test or Total T4 (TT4): This is the most common test. Dogs with failure of the thyroid gland will have a lowered level of the T4 hormone.
  • Determination of serum free T4 levels by RIA (radio immunoassay) or by ED (equilibrium dialysis) is a sensitive and specific way to evaluate a patient for hypothyroidism.
  • Basal serum T3 levels are not an accurate means for evaluating thyroid function; one study found normal serum T3 levels in 74% of hypothyroid dogs.
• TSH stimulation test:
  • The thyroid stimulating hormone help to determine if the thyroid gland or the pituitary gland which controls the thyroid gland is the cause of the hypothyroidism. This test in conjunction with low T4 levels strongly indicate hypothyroidism. Unfortunately TSH is no longer available so we now rely on measuring the dog's TSH level in the blood stream as a part of our analysis.
• TgAA test:
  • The thyroglobulin autoantibodies test for evidence of immune attack of the thyroid. Autoantibodies to Tg and to a much lesser extent, T3 and T4 occur in 30%-50% of dogs with hypothyroidism and are consistent with a diagnosis of lymphocytic thyroiditis.

Treatment 

• The treatment of hypothyroidism, regardless of its cause, is through thyroid hormone supplementation. Usually, if the diagnosis is correct, thyroid supplementation improves the clinical signs associated with the disease within 4-6 weeks. During the first week of therapy, the first improvements are usually seen (an increased mental alertness, greater activity, improved muscle strength and improved appetite). The improvements in the skin take several months to return to normal. All the clinical signs of hypothyroidism are reversible, once treatment is started.
• Levothyroxine (T4) is the initial thyroid supplement of choice. If the patient responds to therapy, once-daily therapy can be tried; however, some patients require continued therapy twice daily. Different brands of levothyroxine frequently have different gastrointestinal absorption so the dosage may be change if the brand is changed.
• The dose of thyroid hormone supplementation should be fine tuned with blood tests which measure the level of thyroid medication in the blood. A blood sample is usually drawn 4 to 6 hours after administration of thyroid medication.
• If secondary bacterial infection of the skin is present, antibiotics are usually prescribed for a minimum of 3 weeks.

Potential complications

• Prolonged administration of an inappropriately high dosage of levothyroxine can cause hyperthyroidism (increased levels of thyroid hormone).
  • Clinical signs of hyperthyroidism include panting, polyphagia (increased appetite), polyuria (increase frequency of urination), polydipsia (increased drinking), anxiety and diarrhea. If these symptoms are observed a T4 level and adjustment of the medication is necessary.

Prognosis

• Prognosis for return to normal function following treatment is excellent in most adult hypothyroid dogs.
• The life expectancy is normal.
• Patients with acquired central hypothyroidism may have a poor prognosis if the condition is secondary to a tumor of destructive process affecting the pituitary gland or the hypothalamus.

References

•  Ettinger SJ, Feldman EC, Canine hypothyroidism, In: Textbook of veterinary internal medicine, W.B. Saunders 2000, p.1442-1448.
• Kantrowitz LB, Peterson ME, Melian C, Nicholas R. Serum total thyroxine, total triiodothyronine , free thyroxine, and thyrotropin concentration in dogs with thyroidal disease. J AM Vet Med Assoc 2001; 219: 765-769.
• Tilley LP, Smith FWK, Hypothyroidism, In: The 5- minute veterinary consult canine and feline, Lippincott Williams& Wilkins third edition 2003.