Structure and Function of the Endocrine System in Cats
Dr. Bari Spielman
Blood tests that measure the amount of circulating hormone in the blood are the most common tests used to detect disorders of the endocrine system. Many different hormones can be measured in the blood, such as cortisol, thyroxine, ACTH, parathyroid hormone, growth hormone and insulin.
What Types of Diagnostic Tests Are Used to Evaluate the Endocrine System?
There are several tests that are helpful in evaluating the endocrine system.
In addition, blood tests have been developed that measure the response of endocrine glands to stimulating hormones. Most of these stimulating tests are timed blood tests. An initial blood sample is taken and the resting level of hormone is measured in that sample. Then a substance is injected into the body, and at some later time (usually within several hours), a second blood sample is taken. The hormone assay is repeated in the second sample to see if the hormone level has changed. Some stimulating tests involve giving different substances in sequence, with several blood samples taken at different intervals. The adrenal glands and thyroid glands are the most common endocrine glands evaluated by using stimulation tests.
Routine serum biochemistry tests, a complete blood count, and a urinalysis can be used to detect clues that the endocrine system is diseased. For example, with hyperthyroidism, several liver and muscle enzyme tests may be elevated. With diabetes mellitus, serum or blood sugar is high. Many endocrine diseases also result in anemia. With Cushing's disease the urine may be very watery or dilute. With diabetes mellitus and acromegaly, sugar is detected in the urine.
Most endocrine glands do not show up on regular plain X-rays, but they may cause abnormalities in other organs that can be detected on X-ray. For example, the liver is often enlarged with Cushing's disease, and sometimes with diabetes mellitus. Elevated calcium levels associated with hyperparathyroidism may result in the formation of kidney or bladder stones that are visible on X-rays.
CT scans and MRIs are very useful in evaluating the endocrine glands. They can often detect enlargement of the glands, distortion in their shape and changes in their location and size. They provide valuable information about the structure of the glands, but do not give much information on the function of the glands.
Radioisotope scans are available for some endocrine glands, such as the thyroid gland. These tests involve the injection of radioactive materials that are taken up by the thyroid gland. These scans provide information about the location and size of the gland, as well as the function of the gland.
Fine needle aspirates and biopsies are sometimes performed on certain endocrine glands. These tests are not usually done on the hypothalamus or pituitary gland, but may be considered for the thyroid gland, liver and kidney. They may also be performed on an enlarged parathyroid gland. Biopsies of the pancreas and adrenal gland must usually be performed during a surgical exploration of the abdomen and must be done cautiously.
Some endocrine diseases are easy to diagnose, while others can be very difficult to confirm. Some endocrine disorders are not diagnosed until multiple tests are performed, sometimes over a period of time. Because some endocrine diseases of cats cause very vague symptoms, they can often be overlooked.