The thyroid gland is an endocrine gland (which releases hormones into the blood). It has the overall shape of a butterfly and is located at the base of the neck. 

The role of the thyroid gland is to produce thyroid hormones (T4 and T3) which pass into the blood to reach all organs.
Thyroid hormones are essential for the metabolism (functioning) of the human body. 
They act on the brain, heart, bones, muscles and thermal regulation and many other tissues that need these hormones to function properly.