Structure and Function of the Thyroid Gland in Cats
Dr. Bari Spielman
What Is the Thyroid Gland?
The thyroid gland is an endocrine gland that produces thyroid hormones that regulate many metabolic processes in the body. Metabolic processes are activities that occur in cells and involve the release of energy from nutrients or using energy to create other substances. Thyroid hormones are important in the maintenance of normal, healthy activity levels of many different organs in the body.
Where Is the Thyroid Gland Located?
The thyroid gland is located in the front of the neck, below the larynx (voice box). The thyroid gland consists of two lobes, one on each side of the windpipe, and each gland is deeply buried in the soft tissues of the neck that surround the voice box.
What Is the General Structure of the Thyroid Gland?
The thyroid gland is an elongated, dark red gland, whose size is variable, depending on the breed and individual animal. It is shaped somewhat like a "bow tie," having two halves or lobes. A normal thyroid gland is usually not palpable (able to be felt) in the cat.
Microscopically, thyroid tissue is made up of two types of cells: follicular cells and parafollicular cells. Most of the thyroid tissue consists of follicular cells, which are composed of millions of tiny saclike structures, called follicles. The follicles store and secrete iodine-containing hormones called thyroxine (T4) and tri-iodothronine (T3). The thyroid requires iodine to produce these hormones. Both T4 and T3 are important regulators of the body's metabolism.
The smaller subset of cells, parafollicular cells, secrete the hormone, calcitonin, which helps to regulate calcium levels in the body by lowering blood calcium.
Two tiny parathyroid glands are associated with each thyroid gland. The parathyroid glands also regulate calcium levels in the body and increase calcium in the blood. The parathyroid glands also help regulate phosphorus levels.
What Are the Functions of the Thyroid Gland?
Although very small, the thyroid gland plays an important role in regulating the body's metabolism and calcium balance. The T4 and T3 hormones stimulate every tissue in the body to produce proteins and increase the amount of oxygen used by cells. The harder the cells work, the harder the organs work. The calcitonin hormone works together with the parathyroid hormone to regulate calcium levels in the body.
The amount of hormone secreted by the thyroid gland is controlled by the amount of thyroid-stimulating hormone (TSH) produced by the pituitary gland. The amount of TSH produced is in turn controlled by the hypothalamus (a tiny cluster of brain cells that sits just above the pituitary gland and transmits messages to the brain).
What Are Common Diseases of the Thyroid Gland?
Hyperthyroidism is a common disorder seen in middle aged and older cats. The thyroid gland becomes overactive, and excessive amounts of the hormone thyroxine (T4) are released. The metabolic rate of many organs increases in this disease, which causes multiple clinical signs including weight loss, ravenous appetite, vomiting, diarrhea, and excessive activity or vocalization.
Hypothyroidism is a rare disorder of cats that arises when the thyroid gland becomes underactive and produces insufficient amounts of T4. Weight gain, sluggishness, and poor hair coat may be seen associated with this disorder.
Thyroid tumors may occur in cats. The most common thyroid tumor of cats is a benign tumor that produces excessive amounts of thyroxine, and produces signs of hyperthyroidism. Malignant tumors of the thyroid are less common.
What Types of Diagnostic Tests Are Used to Evaluate the Thyroid Gland?
There are several tests that are particularly helpful in evaluating the thyroid gland, and most involve measuring levels of T4 or T3.
Serum (blood) T4 levels are usually very elevated in cats with hyperthyroidism. Occasionally T4 is not elevated and more specialized tests are indicated, including measurement of T3, T3-suppression tests, and radioactive iodine scans of the thyroid glands.
In the rare instances of hypothyroidism, both T4 and T3 levels in the blood are low.
Ultrasound examination of the neck/thyroid gland, fine needle aspiration and cytology (microscopically evaluating cells), thyroid biopsy, computed tomography (CT scan) and/or magnetic resonance imaging (MRI) may be helpful to diagnose certain types of thyroid tumors.