Thyroid Tumors and Hyperthyroidism in Dogs

Large neck mass in a dog with
thyroid cancer (carcinoma)

In dogs, thyroid tumors are common, representing approximately 1 to 4% of all canine neoplasms. Most thyroid tumors in the dog are non-functional and do not lead to hyperthyroidism — completely different from the situation in cats, where most cats with thyroid tumors are hyperthyroid (for more information, see links to my related posts, below). 

As opposed to the relatively small, non-invasive and benign thyroid tumors associated with hyperthyroidism in cats, most clinically detected thyroid tumors in dogs are large, invasive tumors associated with thyroid cancer (carcinomas).

Nature of Thyroid Cancer in Dogs
In dogs with thyroid carcinoma, both local invasion of tumor into adjacent structures (e.g., esophagus, trachea, cervical musculature, nerves, or thyroid vessels) and distant metastasis (e.g., lung, liver, or regional lymph nodes) are common.

Approximately 50% of dogs with thyroid carcinoma have documented metastasis at time of diagnosis. During the natural course of disease, 65% to 90% of dogs with untreated disease will develop metastasis to a wide variety of tissue.

Thyroid Tumors and Hyper- and Hypothyroidism
In most dogs with thyroid tumors, their thyroid function remains normal. Up to a third of dogs with thyroid cancer will develop hypothyroidism from destruction of normal thyroid tissue.

Only 10% of thyroid carcinomas in dogs are overactive, producing clinical signs of hyperthyroidism. Although thyroid adenomas producing hyperthyroidism have been described, the incidence of benign thyroid tumors causing hyperthyroidism in dogs is very low. In dogs, a thyroid tumor causing hyperthyroidism should always be presumed to be a carcinoma until proven otherwise. Again, this is in stark contrast to the situation in hyperthyroid cats, in which thyroid carcinoma is rare (less than 5% of in cats).

Clinical Features 
Most dogs with thyroid tumors are of middle- to old-age (greater than 5 years; average age, 10 years). There is no sex predilection. Breeds reported to be at increased risk of developing thyroid tumors include boxers, beagles, Golden Retrievers, and Labrador Retrievers.

Many dogs with thyroid tumors are presented because the owner has noticed an enlargement of the neck. In more than 75% of dogs diagnosed in one survey, either the cervical swelling was the only reason for seeking veterinary care or the thyroid mass was detected by the veterinarian during an examination for another problem.

Because of the large tumor volume and high incidence of both local invasion and distant metastasis, clinical signs such as trouble breathing (dyspnea), cough, hoarseness, alternation in bark, vomiting, poor appetite, and weight loss may be reported, especially in dogs with nonfunctional thyroid tumors. A surprising number of owners, however, fail to report any clinical signs in their dogs.

Of all thyroid tumors in dogs, approximately 10% will become hyperthyroid, and in these dogs, the hyperthyroid state is usually the major reason for examination. Increased thirst and urination are usually the earliest and most predominant signs associated with hyperthyroidism in dogs. Weight loss, despite an increase in appetite, is also common.

Diagnosis
Thyroid imaging or scanning (thyroid scintigraphy) is useful in the evaluation of dogs with thyroid tumors because the procedure can help locate where the thyroid tissue is located (both normal tissue, as well as benign and malignant tumors).  Thyroid imaging can also be performed to help determine the extent of thyroid invasion or metastasis.  

Definitive diagnosis of thyroid carcinoma is best made by either incisional biopsy or surgical excision of the thyroid mass, followed by histological examination of the thyroid tissue. Despite the fact that most thyroid tumors detected in clinical practice are easily palpable, they are not simple to biopsy. Because of the vascular and invasive nature of canine thyroid carcinomas, hemorrhage is a common complication.

Treatment
Treatment of thyroid neoplasia in dogs is dictated by the size of the primary tumor, extent of local tissue invasion, presence of detectable metastasis, presence of hyper- or hypothyroidism, and available treatment options.

Because most clinically detected thyroid tumors in the dog are malignant, treatment is rarely curative. Nevertheless, one should generally advise some form of treatment because palliative relief and increased lifespan can usually be achieved in most dogs with thyroid carcinoma.

Surgical removal of a small thyroid tumor in a dog

Surgery, chemotherapy, cobalt irradiation, and use of radioactive iodine therapy, alone or in combination, may be indicated depending on the individual dog. Medical control of the hyperthyroid state can be achieved by the daily administration of an antithyroid drug such as methimazole or carbimazole (5 to 15 mg/dog, twice, daily), but such medical treatment will not do anything to prevent tumor growth or metastasis.

Suggested Reading:

1. Panciera DL, Peterson ME, Birchard, SJ: Diseases of the thyroid gland. In: Birchard SJ, Sherding RG (eds): Manual of Small Animal Practice (Third Edition), Philadelphia, Saunders Elsevier, 2006;327-342.
2. Peterson ME: Hyperthyroidism and thyroid tumor in dogs. In: Melian C, Perez Alenza MD, Peterson ME, Diaz M, Kooistra H (eds): Manual de Endocrinología en Pequeños Animales (Manual of Small Animal Endocrinology). Multimedica, Barcelona, Spain, 2008;113-125.
3. Mooney CT. Canine hyperthyroidism In: Mooney CT,Peterson ME, eds. BSAVA Manual of Canine and Feline Endocrinology. Quedgeley, Gloucester: British Small Animal Veterinary Association, 2012;86-91.

My Related Posts on Thyroid Disease in Dogs and Cats:

• Hypothyroidism: One of the Top 10 Diagnosed Diseases of Dogs 
• How is Canine Hypothyroidism Diagnosed? 
• How Do We Confirm a Diagnosis of Hypothyroidism in Dogs? 
• How Do We Treat Dogs With Hypothyroidism? 
• Hyperthyroid in Cats: Table of Contents 
• Treatment Options and Considerations for Hyperthyroid Cats 
• More Hyperthyroid Posts, etc 

Hypertension (High Blood Pressure): A Common Problem in Cats

Hypertension is the medical term for high blood pressure, which is a common problem in older cats. In cats, hypertension is commonly found as a complication of other underlying medical conditions (so-called secondary hypertension). However, primary or essential hypertension (i.e., hypertension that develops without any underlying medical disorder) may also be seen in cats (1).

In contrast to people, where essential hypertension is most common, secondary hypertension is more common in cats. The most common cause of secondary hypertension in cats is chronic kidney disease (CKD). After CKD, the next 3 most common causes for hypertension in cats are all hormonal problems (1). These include the following:

• Hyperthyroidism (caused by a tumor of the thyroid gland that oversecretes thyroid hormone).
• Hyperaldosteronism or Conn's syndrome (usually caused by a tumor of the adrenal gland that secretes too much of the hormone aldosterone)
• Diabetes mellitus (caused by lack of sufficient insulin secretion by the pancreas, or resistance to the action of the body's insulin)
• Obesity (yes, fat tissue is the bodies largest endocrine gland, so obesity is a common endocrine disease) 

Damaging Effects of Hypertension

Hypertension is damaging to the body. In general, hypertension becomes an issue when the blood pressure becomes too high for the vessels carrying the blood.

Imagine attaching a garden hose to a fire hydrant. The high pressure from the hydrant would cause the garden hose to explode. Hypertension is similar. When a blood vessel is too small for the pressure on it, it can “explode,” causing internal bleeding. Since the affected vessels are small, the bleeding may not be noticeable, but a lot of little bleeds and a lot of blood vessel destruction can create big problems long-term.

The effects are most serious in certain vulnerable organs, including the eye, brain, and kidneys.

Eyes: The retina (in the back of the eye) is especially at risk in cats with hypertension—sudden or gradual blindness is often the first sign of latent hypertension. Bleeding into the eye and retinal changes such as swelling and detachment can occur. This may result in damage to the cat's vision which is often permanent. In some cats, bleeding into the front of the eye can be seen without the use of special ophthalmology equipment.

Blindness secondary to bilateral retinal hemorrhage

Brain and central nervous system: If a blood vessel ruptures in the brain, the cat may develop neurological signs such as changes in behavior, a wobbly or drunken gait, seizures, dementia, and even coma. In addition to hemorrhage, high blood pressure also increases the risk of embolism: tiny blood clots that form when blood flow is abnormal. These clots can lodge in dangerous locations, such as the brain.

Kidneys: The kidney can also be affected, as it relies on tiny vessels to filter toxins from the bloodstream. Not only is kidney disease the most important cause of hypertension in cats, but CKD also progresses much more rapidly in the presence of high blood pressure.

Even in cats that have hypertension from another cause, high blood pressure damages the kidneys and may increase the risk of kidney failure developing.

Clinical Findings in Feline Hypertension

In many cats, no specific clinical signs of hypertension will be seen until the condition advances to the point when blindness develops from spontaneous bleeding into the eye or retina.

As hypertension is often secondary to another disease, most cats with hypertension will be showing signs attributable to their underlying problem. For example, hyperthyroid cats will generally have weight loss (in spite of an increased appetite) and hyperactivity as the major clinical signs. Cats with CKD or diabetes will generally show an increase in thirst and urination.

Diagnosis of Hypertension in Cats

Early recognition of hypertension is important to minimize the damaging effects of persistently high blood pressure on the eyes and other organs (1,2). Without obvious signs of hypertension, such as blindness, we can diagnose hypertension through screening, as in humans.

If your cat has one of the disorders commonly associated with secondary hypertension, such as renal disease or hyperthyroidism, your veterinarian should check its blood pressure. I recommend that even healthy cats have their blood pressure checked annually, especially if they are over 10 years old. Measuring blood pressure only takes a few minutes, is completely pain-free and is extremely well tolerated by most cats.

A complete eye examination is also essential since ocular disease is common in hypertensive cats. In mildly affected cats, subtle changes to the appearance of the blood vessels at the back of the eye (retina) and to the retina itself may be seen. In more severely affected cats, the changes can be dramatic and include retinal detachment and bleeding into the eye.

Treatment of Feline Hypertension

For any cat diagnosed as having hypertension, our goal of treatment is 3-fold:

1. To reduce the blood pressure using anti-hypertensive drugs
2. To search for an underlying disease, such as kidney disease, which has caused the hypertension. In some cases, for example hyperthyroidism, treatment of the underlying disease may also resolve the high blood pressure.
3. To assess what complications of hypertension are present (such as ocular disease)

Cats vary in their response to anti-hypertensive drugs and some will require dose adjustments to normalize their blood pressure. Once stabilized, hypertensive cats should have their blood pressure monitored every 2 to 4 months to ensure that the pressure remains normal.

References:

1. Jepson RE. Feline systemic hypertension: Classification and pathogenesis. Journal of Feline Medicine and Surgery 2011;13:25-34.
2. Stepien RL. Feline systemic hypertension: Diagnosis and management. Journal of Feline Medicine and Surgery 2011;13:35-43.

How to Feed Cats: Examining the Nutrition of Feral Cats

Paper Review: The Nutrition of Feral Cats 

Estimation of the dietary nutrient profile of free-roaming feral cats: possible implications for nutrition of domestic cats. 

British Journal of Nutrition 2011;106 Suppl 1:S35-48. 

by E.A. Plantinga, G. Bosch, and W.H. Hendriks 

Abstract
Cats are strict carnivores and in the wild rely on a diet solely based on animal tissues to meet their specific and unique nutritional requirements. Although the feeding ecology of cats in the wild has been well documented in the literature, there is no information on the precise nutrient profile to which the cat's metabolism has adapted.

The present study aimed to derive the dietary nutrient profile of free-living cats. Studies reporting the feeding habits of cats in the wild were reviewed and data on the nutrient composition of the consumed prey items was obtained from the literature.

Fifty-five studies reported feeding strategy data of cats in the wild. After specific exclusion criteria, twenty-seven studies were used to derive thirty individual dietary nutrient profiles. The results show that feral cats are obligatory carnivores, with their daily energy intake from crude protein being 52%, from crude fat 46% and from N-free extract (i.e., carbohydrates) only 2%. Minerals and trace elements are consumed in relatively high concentrations compared with recommended allowances determined using empirical methods.

The calculated nutrient profile may be considered the nutrient intake to which the cat's metabolic system has adapted. The present study provides insight into the nutritive, as well as possible non-nutritive aspects of a natural diet of whole prey for cats and provides novel ways to further improve feline diets to increase health and longevity.

My Comments 
In this excellent study (1), the researchers reviewed 27 published studies reporting the feeding habits of feral cats and obtained data on the nutrient composition of the cats’ prey. The results showed that feral cats are obligatory carnivores with a diet high in protein (52% of daily energy) and fat (46% of daily energy) content, but low in carbohydrates (2% of daily energy). The results of this study should be expected since it has long been recognized that cats are strict carnivores, with a high dietary need for protein and fat but no dietary need for carbohydrates (2-5).

The typical prey diet of cats is low in carbohydrate (<10% of metabolizable energy (ME) (6-9). However, most commercially available cat foods are moderate to high in carbohydrate content (> 25 – 55 % ME), partly because of the difficulty in formulating extruded, dry diets that are low in carbohydrate. But higher carbohydrates are also used in cat food diets because cereal is a relatively inexpensive ingredient, and there is demand for lower-cost diets in the pet food market.

I believe strongly that many of the present-day diseases of the domestic cat are related to an "unnatural" diet, one too high in carbohydrates and too low in protein and fat. I believe we should be feeding our cats closer to what they would be eating in the wild, at least if they do not have special nutritional needs because of diseases such as kidney disease. Future research should focus on the value of feeding a natural diet of whole prey as a means of enhancing both feline health and longevity.

The authors should be commended for doing this excellent study and publishing the results. Hopefully, the pet food industry will pay attention.

References:

1. Plantinga EA, Bosch G, Hendriks WH. Estimation of the dietary nutrient profile of free-roaming feral cats: possible implications for nutrition of domestic cats. British Journal of Nutrition 2011;106 (Suppl 1):S35-48.
2. MacDonald ML, Rogers QR, Morris JG. Nutrition of the domestic cat, a mammalian carnivore. Annual Review of Nutrition 1984;4:521-562.
3. Zoran DL. The carnivore connection to nutrition in cats. Journal of the American Veterinary Medical Association 2002;221:1559-1567. 
4. Morris JG. Idiosyncratic nutrient requirements of cats appear to be diet-induced evolutionary adaptations. Nutrition Research Reviews 2002;15:153-168. 
5. Bradshaw JWS. The evolutionary basis for the feeding behavior of domestic dogs (Canis familiaris) and cats (Felis catus). Journal of Nutrition 2006;136:1927S–1931S.
6. Myrcha A, Pinowski J. Weights, body composition and caloric value of post-juvenile molting European tree sparrows. Condor 1970;72:175–178.
7. Vondruska JF. The effect of a rat carcass diet on the urinary pH of the cat. Companion Animal Practice 1987;1:5-9.
8. Crissey SD, Slifka KA, Lintzenich BA. Whole body cholesterol, fat, and fatty acid concentrations of mice (Mus domesticus) used as a food source. Journal of Zoo and Wildlife Medicine 1999;30:222-227.
9. Eisert R. Hypercarnivory and the brain: protein requirements of cats reconsidered. Journal of Comparative Physiology B, 2011;181:1-17.

Antithyroid Drug Treatment Can Induce Remission of Hyperthyroidism in Humans But Not Cats

In humans, Graves' disease is the most common cause of hyperthyroidism in both adults and children (1). Graves' Disease is caused by the patient's immune system producing an antibody that attacks and turns on the thyroid.

One of the goals of treating patients with antithyroid medications is the hope that these drugs may cause the immune system to stop producing the antibody, resulting in the Graves' Disease going into remission (1). In a recent study of children with Graves' disease, up to 50% of these young patients experienced remission of their hyperthyroidism (2).

This is completely different than the situation we see in cats with hyperthyroidism. Cats do not develop Graves' Disease (3-8). Rather, hyperthyroidism in cats is caused by one or more thyroid tumors (see my blog post, Do All Hyperthyroid Cats Have a Thyroid Tumor?). These thyroid tumors are generally benign (adenomas) but can rarely become malignant.

Although cats frequently are treated for prolonged periods with antithyroid drugs to lower their high circulating thyroid levels and manage their hyperthyroidism, these cats will never go into remission (5-9). For more information on the advantages and disadvantages of using antithyroid drugs in hyperthyroid cats, see my blog post, Treating Cats with Hyperthyroidism: Antithyroid Drugs).

References

1. Cooper DS. Antithyroid drugs. New England Journal of Medicine 2005;352:905-917.
2. Léger J, Gelwane G, Kaguelidou F, French Childhood Graves' Disease Study Group. Positive impact of long-term antithyroid drug treatment on the outcome of children with Graves' disease: national long-term cohort study. Journal of Clinical Endocrinology and Metabolism 2012;97:110-119. (summarized here in Clinical Thyroidology for Patients).
3. Gerber H, Peter H, Ferguson DC, et al. Etiopathology of feline toxic nodular goiter. Veterinary Clinics of North America Small Animal Practice 1994;24:541-565.
4. Peterson ME, Ward CR. Etiopathologic findings of hyperthyroidism in cats. Veterinary Clinics of North America Small Animal Practice 2007;37:633-645.
5. Peterson ME: Hyperthyroidism in cats. In: Melian C (ed): Manual de Endocrinología en Pequeños Animales (Manual of Small Animal Endocrinology). Barcelona, Multimedica, 2008; 127-168.
6. Baral R, Peterson ME. Thyroid gland disorders. In: Little, S.E. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders 2012; 571-592.
7. Mooney CT, Peterson ME: Feline hyperthyroidism, In: Mooney C.T., Peterson M.E. (eds), Manual of Canine and Feline Endocrinology (Fourth Ed), Quedgeley, Gloucester, British Small Animal Veterinary Association, 2012;92-110.
8. Peterson ME: Hyperthyroidism in cats, In: Rand, J (ed), Clinical Endocrinology of Companion Animals. New York, Wiley-Blackwell, 2012; in press.
9. Peterson ME, Kintzer PP, Hurvitz AI. Methimazole treatment of 262 cats with hyperthyroidism. Journal of Veterinary Internal Medicine 1988;2:150–157.