The thyroid gland

A multi-lobed gland on the front of the throat, right below the adam's apple. It is responsible for secreting 2 hormones, T3 and T4. These two hormones which we will consider the same (one is the active form) for our purposes control the metabolic rate of the body. The metabolic rate is the "speed" that all the cells in the body burn fuel. Since this is a far reaching function (almost every cell in the body) problems with the thyroid have far reaching consequences.The thyroid produces two primary hormones - thyroxine (T4) and a small amount of triiodothyronine (T3). These hormones circulate in the blood, enter the cells and trigger their metabolic processes. Most T4 is converted into T3 in the tissues.

According to Henry Feldman, NYU School of Medicine:
The purpose of the endocrine system is to maintain the body in its normal state (called homeostasis). The endocrine system works via chemical messengers called hormones. It consists of 4 components:
The hypothalamus at the base of the brain
The Pituitary at the base of the hypothalamus
The endocrine organs (such as adrenal glands)
The target tissues (such as the kidney)
The basic system works as follows (we'll use the thyroid hormones T3 and T4 as an example):
A signal induces the hypothalamus (a section of the brain at the bottom of the brain) to make a releasing hormone (such as Thyrotropin-releasing-Hormone - TRH)
The signal is sent to the pituitary (a small round organ in a pit in the skill behind the nose) which then releases its messenger (such as Thyroid Stimulating Hormone - TSH)
This messenger is received by the endocrine organ (TSH is received by the Thyroid Gland)
The endocrine organ makes its hormone to act on the tissues (Such as T3 and T4)
These hormones feed back to the pituitary to control the level of pituitary hormones (i.e. the more T3 and T4 that is released, the less TSH is made. This is called Negative Feedback.)