Solution for Case 26
Diagnosis: Tentative diagnosis is feline hyperthyroidism based on the classic signalment (older cat) and clinical signs (weight loss, polyphagia, PU/PD, tachycardia, palpable thyroid nodule).
DDx: The main differential diagnoses for a cat with weight loss and PU/PD include Chronic Kidney Disease (CKD) and Diabetes Mellitus. Gastrointestinal disease (e.g., Inflammatory Bowel Disease, GI lymphoma) could also cause weight loss, but less commonly causes significant PU/PD.
Diagnostic tests:
Total Thyroxine (T4) Concentration: A single blood sample was submitted for a total T4 measurement.
Result: 9.8 µg/dL [Reference Range: 0.8–4.0 µg/dL]
This significantly elevated T4 level is diagnostic for hyperthyroidism.
Blood Pressure Measurement: Given the tachycardia and heart murmur, blood pressure was measured using a Doppler.
Result: 175 mmHg systolic [Reference: <160 mmHg]
This result confirms systemic hypertension, a common complication of hyperthyroidism.
Definitive diagnosis: Feline Hyperthyroidism with secondary systemic hypertension.
Treatment: There are four main treatment options for feline hyperthyroidism: medical management, radioactive iodine therapy, therapeutic diet, and surgical thyroidectomy.
This patient was started on medical management to stabilize her condition. She was prescribed Methimazole 2.5 mg orally twice daily. Methimazole works by blocking the synthesis of thyroid hormones in the thyroid gland. The owner was warned to watch for potential side effects, including facial excoriations, vomiting, or lethargy.
For the systemic hypertension, the cat was also started on Amlodipine 0.625 mg orally once daily.
Monitoring Plan: The patient will be re-evaluated in 2 weeks. The recheck appointment will include a physical exam, blood pressure measurement, and a blood draw to check her Total T4, creatinine, and BUN. The creatinine and BUN are monitored closely because treating hyperthyroidism can "unmask" underlying kidney disease by decreasing the glomerular filtration rate (GFR). Once a stable dose of methimazole is achieved (T4 in the low-normal range), she will be monitored every 3-6 months. The owner was also educated on the benefits of radioactive iodine (I-131) therapy as a potential cure once the cat is stable.
