An endocrinologist is a doctor who specializes in the treatment of diseases and disorders that are associated with the endocrine system in the human body. Of course, that begs the question of what exactly the endocrine system is, and what diseases are associated with it.
Endocrinology is a sub-branch of internal medicine, and requires a fairly large amount of education and training.
To become an endocrinologist, you must first go to medical school. After that, you complete three years of post-graduate training in internal medicine. This is followed by a two or three year fellowship in endocrinology.
The endocrine system in the human body is mostly responsible for making a variety of hormones. These hormones are used for a wide range of metabolic functions, most of which are vital for the proper functioning of just about every organ system in the body.
Two of the most common disorders that are treated by an endocrinologist are diabetes and thyroid disease. Diabetes involves the disregulation of insulin. Insulin is made in the pancreas, and used to help control the uptake of glucose in to various cells of the body. There are several forms of diabetes, and an endocrinologist is able to take care of any of them.
Thyroid disease is also treated by endocrinologists. The thyroid is a gland in the neck that makes thyroid hormone. This hormone is used to control a multitude of metabolic functions around the body. There are diseases where a person has too much thyroid, and others where there is a lack. An endocrinologist can take care of all of these conditions.
Of course, there are many other diseases and disorders that an endocrinologist can help with. Pediatric endocrinologists will treat growth hormone disorders. Adults can be treated for acromegaly, and even more rare conditions like Sheehan's syndrome.
People, especially adolescents, can be treated by an endocrinologist if they have problems with reproductive hormones. These problems often become apparent during puberty. Adrenal disease is also taken care of by these doctors. Osteoporosis can be treated by these doctors as well. A completely comprehensive list of diseases that are treated by endocrinologists is not possible - there are just too many.
If the disease is related to a hormone of any kind that is made or used in the body, odds are an endocrinologist is the doctor you will want taking care of you. Endocrinology is largely considered a "thinking" specialty. These doctors do very few actual procedures. Instead, they have tests run (commonly blood tests, but there are others), and spend most of their time forming diagnoses and managing problems. It's interesting, but only if you are the more cerebral type.