Anatomy and physiology of the thyroid gland

The thyroid is a gland at the front of the neck, below the larynx (commonly called the voice box). The thyroid is a little larger than a quarter and is shaped like a butterfly. The thyroid has 2 lobes, one on each side of the trachea (windpipe). The two lobes are connected by a thin bridge of thyroid tissue called the isthmus.

Structure

The thyroid is mostly made up of 2 kinds of cells that make hormones:

follicular cells
C-cells

Other less common cells in the thyroid include structural support cells called stromal cells and immune system cells called lymphocytes.

Function

Follicular cells make the thyroid hormones thyroxine (T4) and triiodothyronine (T3). In order to make these hormones, follicular cells need iodine, a mineral found in some foods and iodized salt. These thyroid hormones control heart rate, body temperature and metabolism (how quickly the body changes food into energy).

Too much of these hormones (called hyperthyroidism) can cause:

rapid and irregular heartbeat
trouble sleeping
nervousness
hunger
weight loss
feeling of being too warm

Too little of these hormones (hypothyroidism) can cause:

lack of energy
increased tiredness
weight gain

The amount of these thyroid hormones released by the thyroid gland is controlled by another hormone called thyroid-stimulating hormone (TSH), or thyrotropin, which is released by the pituitary gland in the brain.

C-cells (also called parafollicular cells) are much less abundant than follicular cells. They secrete the hormone calcitonin. Calcitonin helps control the level of calcium in the blood by slowing down the release of calcium from bones and increasing the excretion of calcium from the kidneys into the urine. However, reduced levels of calcitonin, caused by removal of the thyroid, do not have major effects on the regulation of calcium or bone and mineral homeostasis.

The hormones made by the thyroid have many functions. They help:

regulate the breakdown of carbohydrates, lipids and proteins
promote normal bone formation and maturing of the skeleton
mature the central nervous system in infants
regulate the central nervous system throughout life
maintain normal body temperature

References

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