Canine Hypothyroidism Study Guide

Overview and Clinical Importance

Hypothyroidism is the most common endocrinopathy in dogs, characterized by inadequate production and secretion of thyroid hormones (T4 and T3) by the thyroid gland. This condition affects virtually every organ system due to the fundamental role thyroid hormones play in cellular metabolism, making it a high-yield topic for the NAVLE examination.

The disease typically affects middle-aged dogs (mean age 7 years) and has a strong breed predisposition. Understanding the pathophysiology, clinical presentation, diagnostic approach, and treatment monitoring is essential for successful management of hypothyroid patients.

Anatomy and Physiology

Thyroid Gland Anatomy

The canine thyroid gland consists of two lobes located on either side of the trachea in the cervical region, caudal to the larynx. Unlike humans, dogs do not have an isthmus connecting the two lobes. The gland is composed of follicular cells that synthesize and secrete thyroid hormones and parafollicular C-cells that produce calcitonin.

Hypothalamic-Pituitary-Thyroid Axis

The HPT axis regulates thyroid hormone production through a negative feedback system:

• Hypothalamus: Releases thyrotropin-releasing hormone (TRH) from the paraventricular nuclei
• Anterior Pituitary: TRH stimulates thyrotrophs to release thyroid-stimulating hormone (TSH)
• Thyroid Gland: TSH binds to follicular cell receptors, stimulating synthesis and release of T4 (thyroxine) and T3 (triiodothyronine)
• Negative Feedback: Circulating T4 and T3 inhibit TRH and TSH release to maintain homeostasis

Thyroid Hormone Characteristics

Etiology and Pathophysiology

Classification of Hypothyroidism

Primary Hypothyroidism (greater than 95% of cases)

Primary hypothyroidism results from destruction or dysfunction of the thyroid gland itself. Clinical signs do not appear until approximately 75% of thyroid function is lost.

• Lymphocytic Thyroiditis (50%): Immune-mediated destruction of thyroid follicular cells with lymphocytic infiltration. Characterized by circulating thyroglobulin autoantibodies (TgAA). Heritable component identified in several breeds.
• Idiopathic Thyroid Atrophy (50%): Progressive replacement of thyroid parenchyma with adipose tissue. May represent end-stage lymphocytic thyroiditis. No inflammatory infiltrate present.
• Rare Causes: Thyroid neoplasia (usually causes hyperthyroidism), iatrogenic (surgical thyroidectomy, radioactive iodine), congenital dyshormonogenesis, iodine deficiency (rare)

Secondary Hypothyroidism (less than 5% of cases)

Results from pituitary gland dysfunction leading to decreased TSH secretion. Causes include pituitary neoplasia, pituitary malformations (cystic Rathke's pouch), and pituitary destruction.

Tertiary Hypothyroidism

Results from hypothalamic dysfunction with decreased TRH production. Not documented in dogs.

Breed Predispositions and Risk Factors

High-Risk Breeds

Additional Risk Factors

• Age: Middle-aged dogs (4-10 years), mean age at diagnosis is 7 years
• Sex: No definitive sex predisposition; some studies suggest spayed females and neutered males at higher risk
• Body Size: Medium to large breed dogs more commonly affected

Clinical Signs and Physical Examination Findings

Thyroid hormones affect virtually every organ system, resulting in diverse clinical manifestations. Clinical signs are typically insidious in onset and may be present for months to years before diagnosis.

Metabolic Signs

• Lethargy and mental dullness - Most common presenting complaint
• Weight gain without increased appetite - Due to decreased basal metabolic rate
• Exercise intolerance - Easily fatigued
• Cold intolerance - Heat-seeking behavior, difficulty maintaining body temperature

Dermatologic Signs (Present in 60-80% of cases)

• Bilateral, symmetrical, non-pruritic alopecia - Trunk, lateral thorax, ventral neck, caudal thighs, dorsum of tail
• "Rat tail" appearance - Classic finding due to tail alopecia
• Dull, dry, brittle hair coat - Easy epilation
• Delayed hair regrowth after clipping - Post-clipping alopecia
• Hyperpigmentation - Especially in areas of friction
• Seborrhea (oleosa or sicca) - Scaling, greasy coat
• Recurrent pyoderma and otitis externa - Secondary bacterial infections due to impaired immunity

Myxedema and Skin Thickening

Myxedema refers to the accumulation of glycosaminoglycans (primarily hyaluronic acid) in the dermis, causing non-pitting skin thickening. This is most prominent on the face, producing the characteristic "tragic facial expression" with thickened skin folds above the eyes, drooping upper eyelids, and facial puffiness.

Neurologic Signs (6-29% of cases)

• Peripheral neuropathy - Most common; weakness, ataxia, proprioceptive deficits, hyporeflexia
• Cranial nerve dysfunction - Facial nerve paralysis, vestibular disease, laryngeal paralysis
• Megaesophagus - Rare but important association

Cardiovascular Signs

• Bradycardia (decreased sensitivity to catecholamines)
• Weak peripheral pulses
• Association with dilated cardiomyopathy and atherosclerosis (secondary to hypercholesterolemia)

Reproductive Signs

• Females: Prolonged anestrus, irregular estrous cycles, infertility, abortion, poor litter viability
• Males: Decreased libido, testicular atrophy, decreased sperm production, infertility

Exam Focus: The classic NAVLE hypothyroid dog presentation: Middle-aged Golden Retriever or Doberman with lethargy, weight gain, bilateral symmetrical truncal alopecia, "rat tail," and hypercholesterolemia. Remember that dermatologic signs are NON-PRURITIC unless secondary infection is present!

H - Hair loss (bilateral, symmetrical, non-pruritic)

Y - Yielding energy (lethargy, exercise intolerance)

P - Pudgy (weight gain without increased appetite)

O - Obtunded (mental dullness)

T - Tragic face (myxedema)

H - Hypercholesterolemia (80% of cases)

Y - Yearning for warmth (cold intolerance)

R - Rat tail appearance

O - Overweight despite normal appetite

I - Infections (recurrent pyoderma, otitis)

D - Delayed hair regrowth

Myxedema Coma - Emergency Presentation

Myxedema coma is a rare but life-threatening manifestation of severe, uncontrolled hypothyroidism. It has a mortality rate of 25-60% even with treatment.

Clinical Features

• Profound mental depression to stupor/coma
• Severe hypothermia (cannot maintain body temperature)
• Marked bradycardia and hypotension
• Hypoventilation
• Non-pitting edema (myxedema) of face and limbs
• Hyponatremia and hypoglycemia

Emergency Treatment

• IV Levothyroxine: 5 mcg/kg every 12 hours
• Slow, passive rewarming (avoid rapid rewarming)
• IV fluid therapy with dextrose supplementation
• Ventilatory support if needed
• Consider glucocorticoids (possible concurrent hypoadrenocorticism)

Diagnostic Approach

Critical Concept: Hypothyroidism should be a CLINICAL diagnosis supported by laboratory testing. Never diagnose hypothyroidism based on laboratory results alone. A stepwise diagnostic approach is essential to avoid misdiagnosis.

Step 1: Clinical Suspicion

Only test dogs with clinical signs consistent with hypothyroidism. Testing obese dogs or including T4 in routine wellness panels leads to overdiagnosis.

Step 2: Baseline Laboratory Evaluation

Complete Blood Count

• Mild normocytic, normochromic, nonregenerative anemia - Present in 30-40% of cases

Serum Biochemistry

• Hypercholesterolemia - Present in approximately 80% of cases (most consistent finding)
• Hypertriglyceridemia - Common concurrent finding
• Elevated ALP and CK - May be mildly elevated
• Urinalysis typically unremarkable

Step 3: Thyroid Function Testing

Interpretation Guidelines

• TT4 within reference range: Hypothyroidism very unlikely - no further testing needed
• TT4 low + fT4 low + TSH high: Hypothyroidism highly likely
• TT4 low + fT4 normal: NOT hypothyroid - consider euthyroid sick syndrome
• TT4 low + TSH normal: Equivocal - measure fT4 or consider therapeutic trial

Euthyroid Sick Syndrome (Nonthyroidal Illness)

Critical concept: Many nonthyroidal illnesses cause decreased T4 concentrations in euthyroid dogs. This is the most common cause of FALSE POSITIVE thyroid test results.

Conditions Causing Euthyroid Sick Syndrome

• Any severe systemic illness
• Hyperadrenocorticism (Cushing's disease)
• Diabetes mellitus
• Renal failure, hepatic disease
• Neoplasia

Drugs That Decrease T4 Concentrations

• Glucocorticoids - Suppress TSH and decrease protein binding
• Phenobarbital - Increases hepatic metabolism of T4
• Sulfonamides (especially potentiated) - Can cause true hypothyroidism with prolonged use
• NSAIDs (especially aspirin at high doses)
• Clomipramine, zonisamide

Thyroglobulin Autoantibodies (TgAA)

TgAA can be detected in approximately 50% of hypothyroid dogs and indicate lymphocytic thyroiditis. Important considerations:

• Positive TgAA does not confirm hypothyroidism (dog may still be euthyroid)
• Only 1/3 of TgAA-positive dogs eventually develop hypothyroidism
• TgAA can interfere with TT4 assays, causing falsely elevated readings
• Useful for identifying dogs with early autoimmune thyroiditis

Treatment

Levothyroxine Sodium (L-T4)

Levothyroxine is the treatment of choice for canine hypothyroidism. Veterinary-approved formulations (Thyro-Tabs Canine, ThyroKare) are recommended over human preparations due to differences in bioavailability.

Special Considerations

• Concurrent cardiac disease: Start at 25-50% of standard dose and increase gradually
• Concurrent hypoadrenocorticism: Stabilize Addison's disease FIRST before starting thyroid supplementation
• Concurrent diabetes mellitus: Monitor closely as insulin requirements may change

Therapeutic Monitoring

Timeline for Clinical Improvement

Laboratory Monitoring Protocol

• First recheck: 4 weeks after starting therapy
• Sample timing: Collect blood 4-6 hours POST-PILL to assess peak concentration
• Target post-pill TT4: Upper half of reference range or slightly above
• Target TSH: Should normalize (return to reference range)
• Once stable: Recheck every 6-12 months

Dose Adjustments

• Post-pill T4 below target: Increase dose by 25%
• Post-pill T4 above target with clinical signs of thyrotoxicosis: Decrease dose by 25%
• Post-pill T4 slightly above reference but dog is doing well: No adjustment necessary (dogs tolerate mild oversuplementation)

Signs of Levothyroxine Overdose (Thyrotoxicosis)

• Hyperactivity, restlessness, anxiety
• Polyuria/polydipsia
• Tachycardia, panting
• Weight loss despite good appetite
• Diarrhea

Prognosis

The prognosis for hypothyroid dogs is EXCELLENT with appropriate diagnosis and treatment. Key points:

• Most clinical signs are completely reversible with treatment
• Dogs can have normal quality of life and lifespan
• Neurologic signs may take longer to resolve and may not completely reverse if severe
• Lifelong medication and monitoring required
• Myxedema coma has guarded to poor prognosis