The Kinky Coconut: What Does theThyroid Gland Do?

What is the thyroid responsible for in the human body?
Good question, until I got sick it never really mattered to me all that much. Now though, it's a different story; I need to know all the basics before I can move forward. Let's travel down the information highway together and learn about this butterfly shaped gland.

The thyroid is a vitally important hormonal gland that plays a major role in the metabolism, growth and maturation of the human body. It helps to regulate many body functions by constantly releasing a steady amount of hormones into the bloodstream. More hormones are produced when the body needs more energy, like when it is growing or cold, or during pregnancy.
Your thyroid gland is a small gland, normally weighing less than one ounce, located in the front of the neck. It is made up of two halves, called lobes, that lie along the windpipe (trachea) and are joined together by a narrow band of thyroid tissue, known as the isthmus.

The thyroid is situated just below your "Adams apple" or larynx. During development (inside the womb) the thyroid gland originates in the back of the tongue, but it normally migrates to the front of the neck before birth. Sometimes it fails to migrate properly and is located high in the neck or even in the back of the tongue (lingual thyroid). This is very rare. At other times it may migrate too far and ends up in the chest (this is also rare)

What does my thyroid gland do?

The thyroid gland produces hormones which regulate the body’s metabolic rate as well as heart and digestive function, muscle control, brain development and bone maintenance. Its correct functioning depends on having a good supply of iodine from the diet.
The release of thyroid hormones from the thyroid gland is controlled by thyrotrophin-releasing hormone (TRH) from the hypothalamus in the brain and by thyroid stimulating hormone (TSH) produced by the pituitary gland. This forms part of a feedback loop called the hypothalamic-pituitary-thyroid axis.
The function of the thyroid gland is to take iodine, found in many foods, and convert it into thyroid hormones: thyroxine (T4) and triiodothyronine (T3). Thyroid cells are the only cells in the body which can absorb iodine. These cells combine iodine and the amino acid tyrosine to make T3 and T4. T3 and T4 are then released into the blood stream and are transported throughout the body where they control metabolism (conversion of oxygen and calories to energy).
This makes iodine an important substance for thyroid metabolism. It is a trace element, which means that the body cannot produce it itself, but needs to take it in with food. In the intestine, iodine is taken from the food and enters the blood stream. After a few intermediary steps it is built into the thyroid hormones in the thyroid gland.

What are these T3 & T4 hormones my thyroid gland produces?

Every cell in the body depends upon thyroid hormones for regulation of their metabolism. The normal thyroid gland produces about 80% T4 and about 20% T3, however, T3 possesses about four times the hormone "strength" as T4.
The thyroid gland produces thyroxine (T₄), which is a relatively inactive prohormone and lower amounts of the active hormone, triiodothyronine (T₃). Collectively, T₃ and T₄ are referred to as the thyroid hormones. Twenty percent of the body’s triiodothyronine is made by the thyroid gland; the other 80% comes from thyroxine converted by organs such as the liver or kidneys.
The thyroid gland also produces calcitonin from cells called C-cells. Calcitonin is understood to play a role in regulating calcium levels in the body, but its exact function in humans remains unclear.

Since the thyroid hormones are vitally important, there is usually always a sufficient amount of T3 and T4 in the body. Some of the hormones are stored in the thyroid as droplets; others are tied to carrier proteins in the blood. When the body needs more hormones, T3 and T4 can be released from the proteins in the blood and take their effect. Moreover, the follicles in the thyroid tissue can supply more hormones.
T3 and T4 increase the basal metabolic rate. All body cells then work harder and therefore need more energy. This means:

Body temperature rises
The heart beat becomes stronger and the pulse faster
Food is used up more quickly because energy stored in the liver and muscles is broken down
Brain maturation is promoted (in children)
Growth is promoted (in children)
Activation of the nervous system leads to higher levels of attention and quicker reflexes

The thyroid gland is under the control of the pituitary gland, a small gland the size of a peanut at the base of the brain (shown here in orange). When the level of thyroid hormones (T3 & T4) drops too low, the pituitary gland produces Thyroid Stimulating Hormone (TSH) which stimulates the thyroid gland to produce more hormones. Under the influence of TSH, the thyroid will manufacture and secrete T3 and T4 thereby raising their blood levels. +The pituitary senses this and responds by decreasing its TSH production. One can imagine the thyroid gland as a furnace and the pituitary gland as the thermostat.
Thyroid hormones are like heat. When the heat gets back to the thermostat, it turns the thermostat off. As the room cools (the thyroid hormone levels drop), the thermostat turns back on (TSH increases) and the furnace produces more heat (thyroid hormones).

The pituitary gland itself is regulated by another gland, known as the hypothalamus (shown in the picture above in light blue). The hypothalamus is part of the brain and produces TSH Releasing Hormone (TRH) which tells the pituitary gland to stimulate the thyroid gland (release TSH). One might imagine the hypothalamus as the person who regulates the thermostat since it tells the pituitary gland at what level the thyroid should be set.