Thyroid Function and Dysfunction
The thyroid gland, located at the base of the neck and anterior ventral part of the thorax, is the largest organ in the human body specialized for endocrine function (Greenspan and Baxter 1994; Parangi and Phitayakorn 2011). The butterfly-shaped gland produces iodide-containing hormones with a variety of functions. Thyroid hormones influence growth and development as well as the body’s metabolism, and affect almost all tissues of the body. They stimulate metabolic rate, resulting in increased oxygen consumption, heart contractility, intestinal motility, bone remodeling, and degradation of such substances as cholesterol and other hormones (Neal 2001). Thyroid hormones also stimulate energy expenditure and thermogenesis. High levels promote the breakdown of glycogen to release energy (glucose), whereas low levels favor glycogen synthesis and energy storage. Thyroid hormones also accelerate the intestinal absorption of glucose.
Not surprisingly, persons with thyroid hormone deficiency (hypothyroidism) often feel sluggish, with decreased energy. They tend to experience cold intolerance, slowed cognition and speech, muscle cramps, dry skin, and constipation. In addition, cholesterol degradation is reduced, leading to greater risk of atherosclerosis. In contrast, hyperthyroidism, when thyroid hormone levels are too high, results in an “accelerated” person. Symptoms include inability to tolerate heat, tachycardia (fast heart rate), tremors, weight loss, increased appetite, and diarrhea. Excess thyroid hormone stimulates the catabolism of not only stored fat, but also muscle and bone tissue, leading to muscle weakness and greater risk for osteoporosis.