The endocrine system is composed of ductless glands that secret hormones into the blood stream to act at distant sites.
Hormones can be proteins, polypeptides, amino acids, or steroids.
The most well known hormones are the sex steroids estrogen, produced in the ovaries, and testosterone, produced in the testes. Estrogen and testosterone are also produced in the adrenal glands of both sexes. Other hormones include thyroxin, produced in the thyroid, and insulin, produced in the pancreas. The pituitary and hypothalamus in the brain release a variety of hormones that affect other organs, including the sex glands. From the blood, hormones interact with cells by binding to special proteins called receptors.