This is a medical specialty, but it’s really vague. Here’s the definition from Wikipedia:
Internal medicine or general internal medicine is the medical specialty dealing with the prevention, diagnosis, and treatment of internal diseases.
That could mean anything.
I know an internist at church. I asked him once what that entailed. He said:
We deal with adult diseases. Essentially, anything that’s not kids, or that doesn’t fall into some other specialty, like cancer.
He mentioned that they do a lot of work with Type 1 and 2 diabetes, for example.
Here’s a definition from the Medical University of the Americas.
Internal medicine is a discipline that focuses on the care of adults and involves trained physicians handling a broad range of medical conditions across a variety of organ systems. Often known as internists or general internists, doctors specializing in internal medicine are recognized as being experts in diagnostics, the treatment of chronic illnesses, health promotion, and disease prevention. While some physicians make a career in the practice of internal medicine, other internists go on to specialize in a more focused area of medicine, such as gastroenterology or cardiology.
Here’s a quote from a “patient referral specialist” in a health system:
“We usually start by explaining that internal medicine doctors see patients 18 and over, while family medicine doctors can see patients of all ages, as well as provide many of the services a woman could get from an OB-GYN.”
I think it’s clear that there’s a ton of overlap there. My feeling is that internal medicine deals with chronic conditions that are beyond the scope of a first line family practice doctor.