Endocrine System

The thyroid is a butterfly-shaped gland that wraps around the back of the esophagus. The two lobes of the thyroid are connected by a band of tissue called the isthmus. An external covering of connective tissue separates each lobe into another 20-40 follicles. Between the follicles are numerous blood and lymph vessels in another connective tissue called stroma. The epithelial cells around the edge of the follicles produce the major thyroid hormones.

The major hormones produced by the thyroid are triiodothyronine (T3), thyroxine (T4), and calcitonin. T3 and T4 are iodine-rich molecules that fuel metabolism. The thyroid hormones play several important roles in growth, metabolism, and development. The thyroid of pregnant women often become enlarged in late pregnancy to accommodate metabolic requirements of both the woman and the fetus.

Thyroid hormones accelerate metabolism in several ways. They promote normal growth of bones and increase growth hormone output. They increase the rate of lipid synthesis and mobilization. They increase cardiac output by increasing rate and strength of heart contractions. They can increase respiration, the number of red blood cells in the circulation, and the amount of oxygen carried in the blood. In addition, they promote normal nervous system development including nerve branching.