The thyroid gland is one of the glands of the endocrine system which lies outside of a body cavity.It is a small gland weighing about 1 ounce (28 grams) and is located in the neck, just below the larynx. This gland secretes two hormones: thyroxine and calcitonin (thyrocalcitonin). Thoroxine affects the growth rate and metabolism of all of the body's cells.
It controls reflexes and regulates the rate at which the body produces energy and transforms food into body components. This hormone causes cells to speed up the release of energy from foods as needed by stimulating catabolism (increasing basal metabolism). All body functions depend upon the normal supply of energy, normal thyroid secretion is, therefore, vital to the body. One of the main components of thoroxyne is iodine, an important body building agent. Babies cannot grow properly without iodine. In older people, iodine deficiency causes hair loss, slowed speech, and drying and thickening of the skin. An average adults iodine requirement is only about a millionth of an ounce (0.00003 grams) per day but it is vital to the delicate balance between health and sickness.
Thyroxine also controls temperature. People with overactive thyroid glands tend to feel uncomfortably hot in cool conditions, while those whose thyroid gland is underactive tend to feel cool even on hot days. The other hormone produced by the thyroid gland, calcitonin, tends to decrease the amount of calcium in the blood, the opposite effect of parathormone from the parathyroid glands. Calcitonin functions to help maintain homeostasis of blood calcium. It prevents a harmful excess of calcium in the blood, called hypercalcemia, from developing.