Feline Frenzy: Cracking The Case of A Shrinking Cat

Why do cats with hyperthyroidism lose weight? What does this paradox reveal about the diseases effects on metabolism?

Veterinarians are often presented with “shrinking”  cats. One important disease to consider with weight loss in cats is hyperthyroidism. After this lesson, you will be able to

• identify the location of the thyroid glands
• describe the roles of the thyroid glands and how they affect metabolism
• relate the roles of thyroid to how they increase energy demands for the body
• describe why hyperthyroid cats lose weight

Lesson

Thyroid glands and their function

Cats have two thyroid glands in their necks. The thyroid glands are responsible for generating thyroid hormones; thyroid hormones control the bodies metabolic rate by influencing how cells convert nutrients into energy. Metabolic effects include oxygen consumption, energy production, and heat production.

Thyroid hormone effects on the body

Thyroid hormone affects growth and development, heart and circulatory systems, thermoregulation, nervous system function, and digestive system activity. Thyroid hormones affect growth and development by supporting development of bones, muscles, and other tissues. They influence the cardiovascular system by controlling heart rate and contractility. Thyroid hormones help regulate body temperature by influencing heat production in cells. Thyroid hormones influence the nervous system by increasing alertness and reflexes, and by supporting normal functioning of the nervous system. Finally, thyroid hormones influence digestion and appetite by controlling gastrointestinal motility and overall energy demand.

Hyperthyroidism

Hyperthyroidism is the overproduction of thyroid hormone by the thyroid gland; hyperthyroidism is indicated by oversized thyroid glands in the neck. One or both thyroid glands can be affected, however, it is more common for both to be affected.

Hyperthyroidism is most common in older cats. The average age of affected cats is 13 and only  five percent of cats are under 10 years old.

Signs and Symptoms

Cats with hyperthyroidism may present the following symptoms:

• Increased activity and restlessness
• aggressive or cranky behavior
• increased heart rate
• increased urination
• periodic vomiting
• Increased defecation or diarrhea
• difficulty breathing

The previous listed symptoms, along with increased hormone production, contribute to increased energy demands for cats. Usually cats experience weight loss because they can not consume enough calories and nutrients to keep up with the increased energy demands.

Diagnosing hyperthyroidism

Hyperthyroidism can be detected by the presence of increased thyroid hormone in blood samples. Furthermore enlarged thyroid glands can be felt in the neck.

Hyperthyroidism treatment

Life-long anti-thyroidal medications , surgical removal of thyroid glands, and radioactive iodine treatments are all options for treating hyperthyroidism in cats. The medication treatment takes several weeks to reduce thyroid hormone levels in the body and levels will increase if medication is stopped. Surgical removal can usually be performed without complication. Occasionally cats will require thyroid pills if thyroid levels become too low. Radioactive iodine kills abnormal thyroid cells and spares the normal ones. Radioactive iodine is very effective and safe.

Activity 1 – The roles of thyroid hormone

Thyroid hormone is responsible for regulating many organs and organ systems in cats. Thyroid hormone often up regulates organs within the body.  Try to match the effects that thyroid hormone produces to the organ system it affects:

Talk with your neighbor:

• What did you learn after completion of the activity
• what were the organs affected by thyroid hormone
• How might the effects of thyroid hormone affect the energy demands of cats.

Activity 2 – Hyperthyroidism and Weight-loss

Thyroid hormone produced by the thyroid gland is responsible for up-regulating many things.  The up-regulation increases the bodies calorie demand. Hyperthyroidism results in an excess production of thyroid hormone which further up-regulates organs in the body. Complete the following activity on hyperthyroidism induced weight-loss

• What did you notice about the calories burned vs. the calories consumed?
• How does this activity relate to the previous activity?
• Why would having more calories burned vs. calories consumed result in weight loss?

Assessment

Quick Case Connection: Hyperthyroidism and the Ileal Brake in Cats

Case Study Summary: Whiskers, a 6-year-old cat, has been losing weight rapidly, even though she seems to be eating more than usual. Tests show she has hyperthyroidism, which increases the release of thyroid hormones (T3 and T4). These hormones are speeding up her metabolism, causing her body to burn more energy than she consumes. Additionally, researchers suspect that hyperthyroidism may interfere with the ileal brake mechanism. The ileal brake normally slows digestion when nutrients reach the ileum, helping regulate appetite and energy absorption.

Further exploration

When animals present to a veterinarian in real life, it is common that hyperthyroidism has been present for some time already. As a result, they may have developed diseases and symptoms from thyroid hormones being too high for too long. It is the job of the veterinarian to determine if these additional symptoms need to be treated in addition to hyperthyroidism. Here are some supplemental materials outlining some complications that can happen as a result of hyperthyroidism:

• Because excess thyroid hormone leads to a higher heart rate and increased contractility force, the heart can become more muscular, or hypertrophy, similar to when your biceps add muscle when you more frequently go to the gym. This is normally a good thing in our biceps, but it can be problematic in our hearts. Cats with hyperthyroidism can develop heart failure and other cardiac diseases. Veterinarians may have to prescribe additional medications to combat heart disease in these cats.
  •  Feline heart disease (Cornell University)

• About 1 in 5 cats diagnosed with hyperthyroidism will also have high blood pressure, called hypertension. Hypertension causes increased blood pressure in the entire body including in the blood vessels that supply organs such as the eyes, brain, and kidneys. Impacts in the eyes can range from nothing all the way to blindness. Impacts on the brain can be noticed as a lack of balance, or changed behavior. Hypertension can also damage kidneys, which are normally responsible for filtering waste products out of blood and into the urine. Protein, an important nutrient for cats (and humans!) is sometimes filtered into the urine in cats with hypertension. Veterinarians may have to prescribe additional medication to manage hypertension in cats with hyperthyroidism.

  • Feline hypertension (Cornell University)
• Why might hyperthyroidism lead to diarrhea or increased defecation in cats? The answer to this is not as complicated as you might think. Excess thyroid hormone can lead to increased gut motility, which means that food is moving through the intestines at a faster rate than normal. This can cause more frequent defecation as nothing slows down the food before defecation. Additionally, spending less time in the intestines gives them less time to absorb nutrients and water out of the food. Excess water left in the food causes diarrhea. Usually diarrhea does not require any additional treatment than that used to treat the hyperthyroidism.

Bonus Crossword

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