Anatomy
The thyroid gland (glandula thyroidea) consists of two lobes made up of follicles. These lobes are situated on either side of the trachea, typically at the level of the first three to eight tracheal rings. Under normal circumstances, the thyroid is not palpable, as it is located deep within the neck and has a consistency comparable to muscle tissue in cats.

Function of the Thyroid
The thyroid produces thyroid hormones (triiodothyronine: T3 and thyroxine: T4) from iodine and the amino acid tyrosine. The production of T3 and T4 is regulated by the hypothalamus and the pituitary gland. The hypothalamus secretes thyrotropin-releasing hormone (TRH), which stimulates the pituitary gland to produce thyroid-stimulating hormone (TSH). TSH, in turn, prompts the thyroid to produce T3 and T4. The primary functions of the thyroid include the regulation of metabolism and growth through these hormones.

Causes
Hyperthyroidism in cats is primarily caused by benign neoplasia of the thyroid. In approximately 99.9% of cases, it is a benign tumor, with malignant tumors occurring in only 1 in 1,000 cases. Most cats with thyroid issues present with multiple small proliferations of the thyroid. These proliferations can vary pathologically from hyperplasia to adenoma. In 70% of cases with an overactive thyroid, the condition is bilateral, with both thyroid glands containing a benign tumor. Lingehoeve Diergeneeskunde is the only veterinary clinic in the Netherlands that has been treating cats with thyroid problems using radioactive iodine for many years.

Symptoms of Hyperthyroidism
Typically, a cat with hyperthyroidism is over 10 years of age. Owners may not immediately notice the enlarged thyroid glands. Common symptoms include:

• Weight loss despite a normal or increased appetite
• Hyperactivity
• Behavioral changes (e.g., restlessness)
• Tachycardia
• Hypertension
• Increased thirst and urination
• Panting due to heat intolerance
• Heart failure (hypertrophic cardiomyopathy)
• Diarrhea

Listless Hyperthyroidism
In some cases, cats may exhibit atypical symptoms known as listless hyperthyroidism. These cats may lose weight but, instead of having a high appetite, they may stop eating altogether and become lethargic rather than hyperactive. This presentation may indicate an advanced stage of hyperthyroidism.

Diagnosis
Diagnosis of hyperthyroidism in cats can be based on clinical signs and elevated T4 levels in the blood. Additionally, symptoms of reduced kidney function (elevated urea/creatinine levels) and increased blood pressure may suggest hyperthyroidism. A thyroid scan can further assist in confirming the diagnosis. Lingehoeve Diergeneeskunde is among the few clinics in the Netherlands that offer these scans.

Therapy
There are four treatment options available: medication (oral or transdermal), surgery, dietary management, and radioactive iodine treatment. Each treatment option has its pros and cons, making it essential to tailor the approach to the individual patient. A thyroid scan can help determine the most appropriate treatment for each case. Radioactive iodine treatment is globally regarded as the most effective therapy for hyperthyroidism, and it is offered at Lingehoeve Diergeneeskunde.

For Veterinarians
This page is intended for veterinarians who encounter patients with hyperthyroidism in their practice. If you have a cat presenting with symptoms consistent with hyperthyroidism, more information can be found under ‘Thyroid Problems – Symptoms.’