The thyroid gland (glandula thyreoidea) is situated at the front of the throat. The thyroid cells produce hormones, mainly thyroxine (T4) and a small amount of triiodothyronine (T3). The pituitary, an endocrine gland in the brain, controls the thyroid gland and, in doing so, ensures that the hormones produced end up in the bloodstream. By way of the bloodstream, the thyroid hormones reach all of the cells in all parts of the body.
Thyroid hormone plays a role in the metabolism of virtually all bodily tissues, and it stimulates longitudinal growth of the bones. Before birth and during the first three years of life, thyroid hormone is important for the growth and development of the brain and, after that, for its function. The hormone works as a kind of gas pedal and ensures that your 'engine' runs faster or slower.

A properly functioning thyroid gland produces precisely the right amount of thyroid hormone, day and night. But the thyroid does not always work as it should and can produce and secrete too little or too much thyroid hormone. You can have a thyroid problem at birth, or it can develop in later years. For example, something may have gone wrong during the development and migration of the thyroid or with the synthesis or secretion of thyroid hormone.
The thyroid gland can not be controlled properly if the pituitary is not functioning properly.