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, involving 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 like a "bow tie," having two halves or lobes. A normal thyroid gland may or may not be palpable (felt in the neck with fingertips); this varies in certain breeds of dog.
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 the 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?
Hypothyroidism is a common disorder seen in dogs. The thyroid becomes underactive and produces insufficient amounts of T4. Weight gain, sluggishness, poor hair coat, and other signs of decreased metabolism are commonly seen associated with this disorder.
Hyperthyroidism is rare in the dog, and is most commonly produced by thyroxine secreting tumors. Thyroid tumors may be both benign and malignant in the dog. Tumors that overproduce thyroid hormone in the dog are often malignant.
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 and T3 under different circumstances. In dogs with suspected hypothyroidism, measurement of two forms of T4 and TSH levels is often performed. Low T4 levels and high TSH levels confirm the diagnosis.
When hyperthyroidism is suspected, particularly if due to a tumor, measurement of T4 levels and radioactive iodine scans may be helpful. In addition, 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 also be needed to confirm the presence of a thyroid tumor.