Thyroxine

Thyroxine is the primary hormone produced by the thyroid gland.
Every cell in the body depends upon thyroid hormones for regulation of their metabolism.
The thyroid gland is the biggest gland in the neck. It is situated in the anterior (front) neck below the skin and muscle layers. The thyroid gland takes the shape of a butterfly with the two wings being represented by the left and right thyroid lobes which wrap around the trachea. The sole function of the thyroid is to make thyroid hormone. Thyroid hormone has an effect on nearly all tissues of the body where it increases cellular activity. The function of the thyroid is to regulate the body's metabolism (conversion of oxygen and calories to energy).
In healthy people, the thyroid makes just the right amounts of hormones, which have important actions throughout the body. These hormones regulate many aspects of our metabolism, eventually affecting how many calories we burn, how warm we feel, and how much we weigh. In short, the thyroid "runs" our metabolism. These hormones also have direct effects on most organs, including the heart which beats faster and harder under the influence of thyroid hormones. Essentially all cells in the body will respond to increases in thyroid hormone with an increase in the rate at which they conduct their business.