Feline Hypothyroidism Study Guide

Overview and Clinical Importance

Hypothyroidism in cats is a condition characterized by inadequate production of thyroid hormones (T4 and T3) by the thyroid gland. Unlike in dogs where primary hypothyroidism is common, feline hypothyroidism is rare as a spontaneous condition and most commonly occurs as an iatrogenic complication following treatment for hyperthyroidism. Understanding this distinction is critical for NAVLE success.

The thyroid gland produces thyroxine (T4) and triiodothyronine (T3), which regulate metabolic rate, oxygen consumption, protein synthesis, and affect virtually every organ system. Hypothyroidism results in decreased metabolic function with widespread systemic effects.

Thyroid Anatomy and Physiology

Cats have two thyroid glands (lobes) located in the ventral cervical region, lateral to the trachea at approximately the level of the 4th through 8th tracheal rings. Normal feline thyroid glands measure approximately 10-16 mm long, 3-5 mm wide, and 1-2 mm thick. Unlike dogs, the central isthmus connecting the two lobes is often vestigial or absent in cats.

Hypothalamic-Pituitary-Thyroid Axis

Thyroid hormone secretion is regulated through negative feedback: Thyrotropin-releasing hormone (TRH) from the hypothalamus stimulates the pituitary to release thyroid-stimulating hormone (TSH), which then stimulates thyroid hormone synthesis and secretion. When T4 and T3 levels are low, TSH rises in an attempt to stimulate the thyroid gland, creating the classic pattern of primary hypothyroidism: low T4 with elevated TSH.

Etiology and Classification

Congenital Hypothyroidism (Cretinism)

Congenital hypothyroidism results from defects present at birth affecting thyroid hormone production. It can be classified as goitrous (thyroid dyshormonogenesis with goiter) or nongoitrous (thyroid dysgenesis without goiter).

Clinical Signs of Congenital Hypothyroidism

Radiographic Findings in Congenital Hypothyroidism

• Epiphyseal dysgenesis: Delayed or reduced ossification of epiphyseal cartilages
• Widened growth plates: Inappropriate for age
• Shortened vertebral bodies: Square or widened appearance
• Delayed carpal/tarsal ossification: Bones may not be visible
• Megacolon: Common but unexplained finding

Adult-Onset and Iatrogenic Hypothyroidism

Clinical signs in adult cats are often subtle and nonspecific, making diagnosis challenging. Many cats with mild iatrogenic hypothyroidism show no obvious clinical signs. When present, signs may include:

Exam Focus: Unlike dogs, bilaterally symmetric alopecia does NOT develop in hypothyroid cats. Obesity may develop, especially in iatrogenic hypothyroidism, but is not consistent. The clinical signs in adult cats are often so subtle that they may be mistaken for "normal aging."

Diagnosis

Diagnosis of feline hypothyroidism requires a combination of clinical signs, laboratory findings, and ruling out nonthyroidal illness. A single low T4 value is NOT diagnostic, as nonthyroidal illness can suppress T4 levels.

Laboratory Diagnostic Criteria

Diagnostic Algorithm

• Clinical suspicion: History of hyperthyroid treatment OR clinical signs consistent with hypothyroidism
• Initial testing: Measure T4 and TSH together
• Interpretation: Low T4 + High TSH = Hypothyroidism confirmed
• If equivocal: Repeat testing in 2-4 weeks; consider nonthyroidal illness
• Additional workup: CBC, chemistry, urinalysis to assess for concurrent CKD

Differential Diagnosis for Low T4

• Nonthyroidal illness syndrome (Euthyroid sick): Low T4 but normal TSH
• Chronic kidney disease: Can suppress T4; check TSH to differentiate
• Medication effects: Glucocorticoids, phenobarbital can lower T4
• Severe systemic illness: T4 suppression proportional to illness severity

Hypothyroidism and Chronic Kidney Disease

The relationship between hypothyroidism and kidney function is critically important for NAVLE. Hypothyroidism decreases glomerular filtration rate (GFR), which worsens azotemia in cats with chronic kidney disease.

Monitoring Post-Hyperthyroid Treatment

• Monitor T4 and TSH at 1, 3, and 6 months post-treatment
• Check renal parameters (creatinine, BUN, SDMA, urinalysis) concurrently
• Target T4: 1.0-2.5 mcg/dL (mid-normal range)
• TSH should normalize with adequate thyroid function
• If new azotemia develops with low T4, measure TSH immediately

Treatment

Levothyroxine (L-T4) Dosing in Cats

• Initial dose: 0.05-0.1 mg (50-100 mcg) PO daily OR 75 mcg PO BID
• Alternative dosing: 0.02-0.04 mg/kg/day (10-20 mcg/kg/day)
• Administration: Give on empty stomach; twice daily preferred due to short half-life in cats
• Target T4: 1.0-3.0 mcg/dL (mid to upper normal range)
• Monitoring: Check T4 and TSH 2-4 weeks after dose changes; sample 4-6 hours post-pill for peak

Signs of Levothyroxine Overdose (Iatrogenic Hyperthyroidism)

• Tachycardia
• Increased appetite, thirst, and urination
• Excitability and nervousness
• Weight loss
• Panting

Prognosis