Parathyroid gland

Parathyroid gland (Photo credit: Wikipedia)

The endocrine system is composed of glands throughout the body. These ductless glands release hormones (chemical messengers) into the bloodstream and interact with receptors in our cells in various ways. They activate these receptors and either alter the cell’s existing proteins or instruct the cell in the building of new proteins. Both of these actions create reactions throughout the body.  

Each of the endocrine system hormones has a specific shape that fits into hormone receptors on the target organs or glands.  There are three general classes (groups) of hormones. These are classified by chemical structure, not function.
Steroid hormones 
Amino acid derivatives (like epinephrine) which are derived from amino acids, especially tyrosine
• Peptide hormones (like insulin) which are the most numerous group of hormones

The endocrine system regulates all biological processes in the body from conception through adulthood and into old age, including the development of the brain and nervous system, the growth and function of the reproductive system, as well as blood sugar levels. The female ovaries, male testes, hypothalamus, pituitary, thyroid, parathyroids, thymus, adrenals and pancreas are the major glands of the endocrine system.

Here is an image showing the major glands of the endocrine system:

Endocrine Glands