Veterinary Clinical Medicine-II

CLINICAL SIGNS AND CLINICO-PATHOLOGICAL CHANGES OF HYPOTHYROIDISM

Clinical signs

• Clinical signs of hypothyroidism may be non-specific and insidious in onset. Common clinical signs attributable to decreased metabolic rate include lethargy, mental dullness, weight gain, unwillingness to exercise and cold intolerance. Obesity occurs in approximately 40% of hypothyroid dogs.

Dermatological changes

• Dermatologic changes occur in 60% to 80% of hypothyroid dogs. Common findings include dry scaly skin, changes in hair coat quality or color, alopecia, seborrhea and superficial pyoderma. Hyperkeratosis, hyperpigmentation, comedone formation, hypertrichosis, ceruminous otitis, poor wound healing, increased bruising and myxedema may also occur.
• Alopecia is usually bilaterally symmetric and is first evident in areas of wear and tear such as the lateral trunk, ventral thorax and tail. The hair is often brittle, easily epilated and loss of under-coat may result in a coarse appearance or puppy – like hair coat.
• Signs of decreased metabolic rate in conjunction with dermatologic abnormalities should increase suspicion of hypothyroidism.
• Hypothyroid dogs are predisposed to recurrent bacterial infections of the skin such as folliculitis, pyoderma and furunculosis.
• Malassezia spp infections and demodicosis are associated with hypothyroidism. Pruritus may occur with concurrent infection.

Reproductive abnormalities

• Female reproductive abnormalities attributed to hypothyroidism include prolonged interestrous interval, silent estrus, failure to cycle, spontaneous abortion, low-birth weight litters, uterine inertia and stillborn puppies.
• Inappropriate galactorrhea apparently due to hyperprolactinemia has been reported in sexually intact hypothyroid bitches.
• Male reproductive problems attributed to hypothyroidism include low libido, testicular atrophy, hypospermia and azoospermia.

Nervous system abnormalities

• Both the peripheral and central nervous systems may be affected by hypothyroidism. Peripheral neuropathy is the best documented neurologic manifestation. Affected dogs have exercise intolerance, weakness, ataxia, paralysis, deficits of conscious proprioception and decreased spinal reflexes.
• Unilateral lameness reported in hypothyroid dogs may be a manifestation of generalized neuromyopathy.
• Dysfunction of multiple cranial nerves and abnormal gait and postural reactions.
• In myxedema, coma, profound mental dullness or stupor is accompanied by non pitting edema, hypothermia with a lack of shivering, bradycardia, weakness and inappetence.
• Abnormalities of the cardiovascular system such as sinus bradycardia, weak apex beat, low QRS voltages and inverted T waves occur in hypothyroid dogs. Reduced left ventricular pump function has also been documented.
• Ocular abnormalities reported in canine hypothyroidism include corneal lipidosis, corneal ulceration, uveitis, lipid effusion into the aqueous humor, secondary glaucoma, lipemia, retinitis, retinal detachment and keratoconjunctivitis sicca.
• Congenital hypothyroidism results in mental retardation and stunted disproportionate growth due to epiphyseal dysgenesis and delayed skeletal maturation. Affected dogs are mentally dull and have large broad heads, short thick necks, short limbs, macroglossia, hypothermia, delayed dental eruption, ataxia and abdominal distention. Dermatologic findings are similar to those seen in the adult hypothyroid dog. Other clinical signs may include gait abnormalities, stenotic ear canals, sealed eyelids and constipation.

Clinicopathologic changes

• Results of hemogram, biochemical panel and urinalysis often support a diagnosis of hypothyroidism.
• A mild non regenerative anemia occurs in 30% of hypothyroid dogs
• Fasting hypercholesterolemia occurs in 75% of hypothyroid dogs where as hypertriglyceridemia occurs in up to 88% cases
• In rare cases hyperlipidemia may lead to atherosclerosis.