The thyroid is about 5 cm-long, butterfly-shaped gland weighing less than an ounce. Located in the front of the neck below the larynx, or voice box, it is composed of two lobes, one on each side of the trachea or windpipe. The lobes are joined together by a narrow band of tissue called the isthmus. The thyroid is one of the glands that make up the endocrine system. The endocrine glands produce, store, and release hormones into the bloodstream that travel through the body and direct the activity of the body’s cells. Thyroid hormones regulate metabolism—the way the body uses energy—and affect nearly every organ in the body. The thyroid gland makes two thyroid hormones, triiodothyronine (T3) and thyroxin (T4).
Thyroid hormones affect metabolism, brain development, breathing, heart and nervous system functions, body temperature, muscle strength, skin dryness, menstrual cycles, weight, and cholesterol levels.
A third hormone produced by the thyroid gland, calcitonin, is not considered a thyroid hormone as such, but affects calcium levels in the blood and controls the build-up of calcium in the bones.
Thyroid hormone production is regulated by thyroid-stimulating hormone (TSH), which is made by the pituitary gland. Located in the brain, the pituitary gland is the “master gland” of the endocrine system.