Veterinary Clinical Medicine-II

HYPOTHYROIDISM

• Hypothyroidism, a condition in which there are low levels of thyroid hormones in the blood, is the most commonly diagnosed endocrine disorder in dogs but rare in cats.
• Hypothyroidism is the result of decreased production of thyroxine (T4) and triiodothyronine (T3) by the thyroid gland.
• Common causes of Hypothyroidism in dogs include
  • Lymphocytic thyroditis
  • Idiopathic follicular atrophy
• Many cases of hypothyroidism have a genetic basis and many breeds are prone to developing hypothyroidism.
• Hypothyroidism may result from dysfunction of any part of the hypothalamic- pituitary - thyroid axis.
• Canine thyroiditis is believed to be immune mediated, but the immunologic and molecular pathogenesis has not been well characterized.
• Because thyroid hormone influence the functions of many organs, hypothyroidisim is considered in the differential diagnosis of a wide range of problems.

Clinical signs

• Clinical signs attributable to decreased metabolic rate include lethargy, mental dullness, weight gain, unwilling to exercise and cold intolerance.
• Obesity occurs in approximately 40% of hypothyroid dogs.
• Common dermatological findings include bilateral alopecia, spares the extremities, hairs easily epilates, alopecia on the bridge of the nose and tail (rat tail), dry dull brittle hair coat,dry scaly skin, changes in hair coat quality or color, alopecia, seborrhea and superficial pyoderma, hyperkeratosis, hyperpigmentation, cereminous otisis, poor wound healing and Myxedema.
• Hypothyroid dogs are predisposed to recurrent bacterial infections such as folliculitis, pyoderma and furunculosis.
• Variable pigmentation to the skin and coat.
• Seborrhoea, poor hair growth after clipping and hypertrichosis- a rare finding especially in Boxer and Irish Setter.

Diagnosis

• Over 50% of hypothyroid dogs have high blood cholesterol and often increased triglycerides and lipoproteins. Thyroid hormones commonly measured include total T4, total T3 and free T4.
• Skin Biopsy reveals non-specific endocrine changes- follicular keratitis, follicular dilatation, telogenisation of hair follicles, evidence of pyoderma , malazessia and demodicosis.
• TSH response test is a test of thyroid gland reserve and is the criterian standard for diagnosis of canine hypothyroidism. TRH response test is sometimes used to evaluate thyroid function in dogs.
• Scintigraphy and thyroid ultrasonography may also be useful in evaluation of dogs with suspected hypothyroidism.

Treatment

• The initial treatment of choice, regardless of the underlying causes of hypothyroidism, is synthetic sodium l-thyroxine (T4).
• A dose of 0.02mg/ kg every 24 hrs. normalizes TSH concentration in most dogs; higher doses (0.04m/kg q 12 hr) are required to consistently normalize T3 concentration.
• Adequacy of therapy should be confirmed by therapeutic monitoring 4 to 8 weeks after beginning l-thyroxine supplementation.
• An incorrect diagnosis of hypothyroidism is the most common reason for treatment failure.
• Diseases such as hyperadrenocorticism, atopy and flea hyper sensitivity may have clinical signs similar to those of hypothyroidism and may be associated with decreased thyroid hormone concentrations.
• Prognosis for return to normal function following treatment is excellent in most adult hypothyroid dogs. Prognosis in myxedema coma is dependent on early recoginition.