Hyperthyroidism is the most common endocrinopathy in domestic cats and the second most common disease in cats over 10 years old, after chronic kidney disease. It is caused by hypersecretion of thyroid hormones (T3 and T4) due, in most cases (98%), to benign nodular hyperplasia or adenoma of one or both thyroid glands. Thyroid carcinoma is rare (<2%).
Why Does Hyperthyroidism Appear in Cats?
The exact cause of feline hyperthyroidism remains under investigation. Several risk factors have been proposed:
- Canned food consumption: several epidemiological studies have associated prolonged consumption of canned wet food (especially seafood and fish flavors) with higher prevalence of hyperthyroidism. The proposed mechanism involves endocrine disruptors (phthalates, BPA) released from the inner lining of cans.
- Iodine deficiency or excess: iodine is essential for thyroid hormone synthesis; both chronic deficiency and excess can alter thyroid function.
- Exposure to brominated flame retardants (PBDE): present in textiles, carpets and upholstery, these are known thyroid disruptors. Cats living in environments with these substances have higher prevalence of hyperthyroidism.
- Breed predisposition: Siamese and Himalayan cats have lower risk than other breeds, suggesting a genetic component.
Symptoms: The Cat That Keeps Eating but Loses Weight
The classic clinical presentation is an older cat with progressive weight loss despite having a voracious appetite (polyphagia). Other frequent signs include:
- Hyperactivity, nervousness, excessive vocalization (especially at night).
- Polyuria and polydipsia (urinating and drinking a lot).
- Intermittent vomiting and diarrhea.
- Unkempt, dull coat with coarser hair.
- Tachycardia (heart rate >220 bpm) and heart murmur.
- Palpable mass in the ventral cervical region (goiter) in 80-90% of cases.
Hyperthyroidism can mask underlying chronic kidney disease: excess thyroid hormones increase renal blood flow and glomerular filtration rate, artificially "improving" creatinine values. When treating hyperthyroidism, creatinine may rise, revealing hidden CKD. It is therefore essential to evaluate kidney function before and after starting treatment.
Diagnosis
Diagnosis is confirmed by measuring total T4 (TT4) in serum. A TT4 >4 µg/dL in a cat with compatible signs is diagnostic. In borderline cases (T4 at the upper limit of normal with symptoms), measurement of free T4 by equilibrium dialysis or the T3 suppression test may be necessary.
Thyroid scintigraphy (when available) allows localization of functioning thyroid tissue, detection of intrathoracic ectopic tissue and is essential before treatment with radioactive iodine.
Treatment Options: Four Approaches
1. Medical treatment with methimazole/carbimazole: methimazole (2.5-5 mg/12h oral or transdermal) inhibits thyroid hormone synthesis. It is the most widely used treatment in Spain for its accessibility. It requires lifelong administration and periodic monitoring for adverse effects: facial pruritus, vomiting, hepatotoxicity and, rarely, bone marrow suppression.
2. Radioactive iodine (I-131): considered the treatment of choice for its high efficacy (90-95% cure with a single dose) and absence of systemic adverse effects. It selectively destroys hyperfunctioning thyroid tissue. Requires hospitalization in an authorized facility for 1-3 weeks until radioactivity levels are safe. Not available everywhere, but available in specialized centers in Spain.
3. Surgery (thyroidectomy): effective but with risk of postoperative hypocalcemia from damage to the parathyroid glands. Less used since the availability of I-131.
4. Iodine-restricted diet (Hill's y/d): controls hyperthyroidism by limiting iodine availability for hormone synthesis. Requires it to be the only food the cat receives (no treats or other foods). A valid option for owners who cannot medicate their cat.
