The thyroid gland is often described as a “butterfly-shaped” gland because it has two lobes that look like butterfly wings on either side of the trachea that are connected to each other by a central isthmus, which lies across the anterior portion of the neck (the front) at the base of the neck. It is only about two inches in length and about an inch in height, making it the size of a large butterfly and often difficult to feel unless it is enlarged. Not everyone has an isthmus.
The thyroid gland lies just beneath the thyroid cartilage, which in males is known as the “Adam’s apple”. Its main function is to control the metabolism of the cells the body. It is a typical endocrine gland, meaning that its function is mediated by the release of hormones into the bloodstream that act on distant sites of the body. The two main thyroid hormones released by the thyroid gland include thyroxine (T4) and triiodothyronine (T3).
The levels of T4 and T3 are tightly regulated in the body as too much or too little of the hormones will cause symptoms connected to an imbalance in the body’s metabolism. Both the hypothalamus in the brain and the pituitary gland, located beneath the brain near the hypothalamus, are responsible for the amount of T3 and T4 that are released via a feedback loop—something very common in the endocrine system.
The thyroid feedback loop looks like this: The hypothalamus makes thyrotropin-releasing hormone or TRH into the hypothalamic-pituitary portal, which connects the hypothalamus to the pituitary gland. This triggers the anterior pituitary gland to release thyroid stimulating hormone or TSH. TSH carries the signal to the thyroid gland and binds to TSH receptors on the cells of the gland. The binding of the receptor causes T3 and T4 to be released. When the levels are too high, a signal is sent to the hypothalamus, telling it to turn off production of TRH. TSH levels fall and so do the thyroid hormone levels.
The thyroid hormones are crucial to every cell of the body by regulating the speed of cellular metabolism. In the intestinal tract, they control how quickly food is digested. In the heart, they control how fast the heart beats and how strong the muscle cells are. They regulate the speed of nerve cells. This is why an underactive thyroid gland can lead to a slow heartbeat, weight gain, and constipation. An overactive thyroid gland, on the other hand, may cause tachycardia, anxiety, and diarrhea.
The metabolic processes that are directly affected by the thyroid gland and its hormones include body weight, muscle strength, cholesterol levels, gastrointestinal motility, brain function, peripheral nervous system function, menstruation, body temperature, and heart rate. The symptoms of hypothyroidism involve an underactivity of these bodily functions while the symptoms of hyperthyroidism involve an overactivity of these bodily functions.