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Sudden Weight Loss in Cats and Hyperthyroidism

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Layne has fostered kittens and worked in animal shelters for over eight years.

Why is my cat losing weight?

Why is my cat losing weight?

Why Is My Cat Losing Weight?

Cats may lose weight for a number of reasons. Some cats lose weight due to old age, injury, depression, or even dental issues. Some cats are simply picky and just refuse to eat when you change their food up on them! Before jumping to conclusions, it's important to check with your vet. There can be a number of reasons for weight loss in cats, however, one of the most common causes is feline hyperthyroidism.

What Is Hyperthyroidism?

Hyperthyroidism is the most common endocrine disease in middle-aged and older (or geriatric) cats; the cause of the disease is often unknown. The disease is characterized by abnormal cell growth on one or both thyroid lobes. It disrupts hormone levels essential for regulating normal organ function, skeletal and muscular growth, and metabolism.

The thyroid gland sits in front of the neck and its lobes are positioned on either side of the cat’s trachea. The gland is responsible for producing and releasing thyroid hormones that circulate in the bloodstream.

Video: Radioactive Iodine Treatment for Cats

Hyperthyroid Symptoms in Cats

  • Increased metabolic rate
  • Weight loss despite increased hunger
  • Hyperactivity
  • Increased urination and thirst (polyuria and polydipsia)
  • Vomiting and diarrhea
  • Nervousness and excitability

In chronic cases, the following may be present:

  • Increased heart rate (secondary tachycardia)
  • Cardiac murmurs
  • High blood pressure (hypertension)
  • Enlarged heart (cardiomegaly)
  • Excess fluid in lungs (pulmonary edema)
  • Excess fluid around the lungs (pleural effusion)

How Will I Know If My Cat Has a Thyroid Problem?

Most patients upon physical examination exhibit benign thyroid tumors, whereas a small percentage may have malignant thyroid tumors. A cat’s signalment (age, breed, sex), health history, physical exam findings, and lab findings may reveal clinical and biochemical signs suggestive of unhealthy thyroid levels.

Thyroid hormones impact how your cat metabolizes proteins, carbohydrates, and fatty acids, so if excessive amounts of thyroid hormone are present, protein catabolism or the break down of essential amino acids will occur, leading to wasting.

A physical exam in chronic hyperthyroid cases may reveal secondary tachycardia (increased heart rate), cardiac murmurs, hypertension (high blood pressure), and other disease. Radiographs of the thorax may show cardiomegaly (enlarged heart), pulmonary edema (excess fluid in the lungs), or pleural effusion (excess fluid around the lungs). The disease requires diagnosis, monitoring, and curative or palliative management.

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How Is Hyperthyroidism in Cats Diagnosed?

There are three common methods that a vet uses to diagnose cat hyperthyroidism:

  • Physical exam (palpation)
  • 3D chest x-rays
  • T4 serum test (blood sample)

In the disease process of hyperthyroidism, nodules of the thyroid continually grow and secrete abnormal amounts of hormone. Thyroxine or T4 concentration is used in sensitive diagnostic testing to determine whether or not a cat is hyperthyroid. T4 levels are diagnostic in over 90% of cats. Some 10% of cats with hyperthyroidism have otherwise normal serum T4 concentrations. T4 ranges fluctuate out of normal parameters (over days rather than hours), and diagnostics may need to be repeated. 3D chest radiographs are also used to confirm metastatic thyroid cancers and to confirm a diagnosis coupled with clinical findings.

Treatment for Hyperthyroidism in Cats

  • Methimazole: Methimazole is a common anti-thyroid medication used to block thyroid hormone synthesis and decrease the hormonal levels circulating within the body; hormone concentrations may fall within normal range proceeding a month of treatment.
  • Surgery or Thyroidectomy: A thyroidectomy of one or both lobes of the gland is another alternative method of treatment for hyperthyroidism coupled with lifelong hormonal therapy. A thyroidectomy involves removal of the glands via surgery.
  • Dietary Management: Dietary management for hyperthyroidism may be effective by limiting iodine levels and can reverse the disease process. Low-iodine nutrition reduces T4 hormone production and simultaneously aids in keeping urine at an ideal pH.

Does Radioactive Iodine Therapy Work?

Radioactive iodine is the choice treatment for hyperthyroidism and often performed at specialty practices and licensed facilities. Abnormal thyroid tissue absorbs the radioactive 131-I and subsequently destroys the tissue.

A single dose of radioiodine often restores thyroid function without inducing hypothyroidism. Doses are determined based on the following three margins:

  • Uptake by the gland
  • Rate of release of radioiodine from the gland
  • The mass of the gland.

Radioiodine is effectively administered intravenously, subcutaneously, or orally. Radioiodine given subcutaneously is usually found to be effective within six months of treatment. Aqueous sodium iodine I-131 used for oral administration is sterilized and then diluted in a syringe with 0.9% sodium chloride for administration under the skin.


With adequate veterinary care and management, the prognosis for feline hyperthyroidism is generally good. Severe cases and untreated cases may prove to be fatal, and that's why it's important to consider your options and talk to your veterinarian.


  • Bassert, Joanna M., and Thomas Colville. “Clinical Anatomy and Physiology.” Laboratory Manual for Veterinary Technicians. Missouri: Mosby, 2009.
  • Bassert, Joanna M., and Thomas Coleville. “Clinical Anatomy and Physiology for Veterinary Technicians.” 2nd ed. Missouri: Mosby, 2009.
  • Bassert, Joanna M., and John A. Thomas. “McCurnin’s Clinical Textbook for Veterinary Technicians.” 8th ed. Missouri: Saunders, 2014.
  • Becker, David V., and Mark E. Peterson. “Radioiodine treatment of 524 cats with hyperthyroidism.” JAVMA 1 December 1995: 207.11 1422-1428.

This article is accurate and true to the best of the author’s knowledge. It is not meant to substitute for diagnosis, prognosis, treatment, prescription, or formal and individualized advice from a veterinary medical professional. Animals exhibiting signs and symptoms of distress should be seen by a veterinarian immediately.

© 2018 Laynie H

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