What is metabolism? Metabolism is the chemical activity that occurs in cells, releasing energy from nutrients or using energy to create other substances, such as proteins. The basal metabolic rate (BMR) is a measurement of energy required to keep the body functioning at rest. Measured in calories, metabolic rates increase with exertion, stress, fear, and illness.

The thyroid gland is located in the front of the neck, below the larynx (voice box). The small, two-inch gland consists of two lobes, one on each side of the windpipe, connected by tissue called the isthmus.

The thyroid tissue is made up of two types of cells: follicular cells and parafollicular cells. Most of the thyroid tissue consists of the follicular cells, which secrete iodine-containing hormones called thyroxine (T4) and triiodothyronine (T3). The parafollicular cells secrete the hormone calcitonin. The thyroid needs iodine to produce the hormones.

Functions of the thyroid gland:
The thyroid 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.

Levels of hormones secreted by the thyroid are controlled by the pituitary gland's thyroid-stimulating hormone, which in turn is controlled by the hypothalamus.