Diseases of the Endocrine System

Chapter 3 Diseases of the Endocrine System

The cells of the body must maintain homeostasis (equilibrium) with their internal environment because the chemical processes conducted at the cellular level can occur only under specific conditions. The endocrine system works together with the nervous system to achieve a stable internal environment; each system may work independently or the systems may act together to accomplish this task. The hypothalamus and the pituitary gland regulate the release of chemical messengers called hormones (chemicals secreted directly into the bloodstream) to signal target cells to perform certain functions in response to changes in homeostasis. As hormone concentrations in the blood increase, the system is signaled to reduce hormone production; this negative feedback system works much the same way as a thermostat controls the heating system in your home.

Ideally, the glands that regulate the release and balance of hormones work continuously to maintain homeostasis. When one or more of these glands work ineffectively, incorrectly, or not at all, the potential for problems develops. The endocrine glands most commonly involved in diseases of small animals are the thyroid gland, adrenal gland, pancreas, parathyroid gland, and gonads (testes and ovaries) (Fig. 3-1).

Diseases that affect the pituitary gland or the hypothalamus cause multiple irregularities in the body; they are discussed in this chapter only in the context of their relation to other syndromes.


The thyroid gland is located in the ventral cervical region along the lateral margins of the trachea. The gland is not usually palpable in a healthy animal. The gland is composed of follicles that produce the thyroid hormones triiodothyronine (T3) and tetraiodothyronine (T4). These hormones are produced by the follicular cells and stored in the gland until they are needed by the body. A third hormone, calcitonin, is produced by the parafollicular cells in the thyroid gland. Calcitonin acts to increase calcium deposition within bone to decrease blood calcium concentration, whereas T3 and T4 function to control all of the body’s metabolic processes.

Two common diseases of small animals involve the thyroid gland (Fig. 3-2): hypothyroidism (insufficient thyroid hormone) and hyperthyroidism (excessive thyroid hormone). Hypothyroidism is most commonly seen in the canine (1/156-500 animals). Hyperthyroidism is seen primarily in the cat.

Hypothyroid Disease

Primary acquired hypothyroidism is the most common type of hypothyroid disease seen in the dog and usually follows thyroid atrophy or lymphocytic thyroiditis. Central acquired hypothyroidism is rare and develops when the pituitary or hypothalamus is diseased.

Hyperthyroid Disease

Hyperthyroidism is the most commonly seen endocrine disorder in cats; however, it is rarely seen in the dog except as a result of neoplasia. The disease was first documented in the late 1970s to early 1980s, with incidence increasing steadily. The excess of both T3 and T4 results in a multisystemic disease seen primarily in older cats. Bilateral thyroid gland enlargement occurs in approximately 70% of cases and is the result of a functional thyroid adenoma. Thyroid carcinoma is rarely seen in the cat, comprising only 1% to 2% of all cases of hyperthyroidism.


Diabetes Mellitus

Cells need glucose as a fuel. Through the processes of glycolysis, the citric acid cycle, electron transport, and oxidative phosphorylation, glucose is chemically converted into energy in the form of adenosine triphosphate, carbon dioxide, and water. It is therefore important for the body to regulate the concentration of glucose in circulation. Levels must be kept within certain limits to assure that adequate fuel is always available for energy production. The endocrine pancreas aids in this regulation; β cells in the pancreatic islets (islets of Langerhans) produce the hormone insulin, which facilitates the entry of glucose into the cell for the process of glycolysis. Diabetes mellitus results when these β cells stop producing insulin in adequate amounts or when the cells in specific body tissues become resistant to the action of insulin. The incidence of diabetes mellitus in the dog and cat is reported to be between 1 in 100 and 1 in 500. The cause of the disease is unknown, although chronic pancreatitis, immune-mediated disease, and hereditary predisposition have been suggested as possible causes.

Almost 100% of dogs and about 50% of cats will have insulin-dependent diabetes (type I) at examination. As many as 50% of presenting cats will have non–insulin-dependent diabetes (type II), which does not require insulin therapy.

Therapy for diabetes mellitus in most animals includes dietary regulation (usually a high-fiber diet) and daily insulin replacement. In non–insulin- dependent cats, drug therapy and diet restriction are somewhat successful in managing the disease.

The type of insulin chosen for therapy depends on the severity of the disease and animal needs. Treatment should be tailored to the species of animal involved. (Feline insulin most resembles beef insulin, and canine insulin resembles pork and human insulin.) Although it would be ideal to match the structure of each species’ insulin, it is not easy to do this. Beef and pork insulin and their combinations have virtually disappeared from the market since the late 1990s; Vetsulin has taken their place as the most available insulin.

Animals with diabetes whose sugar concentration remains uncontrolled may become ketotic. Cells begin to use fat as fuel for energy production, yielding ketone bodies that accumulate in the blood. Acidosis, dehydration, and electrolyte imbalances can occur as a result of ketosis.


Insulin Therapy


These forms of insulin recently have been removed from the market by Eli Lilly and Company. Animals that must be switched to another form of insulin should be monitored carefully. It is recommended that doses be reduced when switching until the animal’s glucose levels stabilize.

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Aug 31, 2016 | Posted by in GENERAL | Comments Off on Diseases of the Endocrine System

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