South Nassau Communities Hospital in Oceanside on Long Island, near New York City, said patients may have received insulin from a pen reservoir - not a single-use disposable needle - that could have been used on more than one patient.
"The risk of infection from this is extremely low," the hospital said in a statement, adding it was recommending patients "be tested for hepatitis B, hepatitis C and HIV."
When asked if anyone was confirmed to have been infected a hospital spokeswoman said "not to my knowledge."
The pen-shaped insulin injector devices are often used by hospitals to give the hormone to patients and contain a reservoir or cartridge, the Centers for Disease Control and Prevention say.
The pens should be limited to one patient because regurgitation of blood into the insulin cartridge can occur after injection, creating a risk of blood-borne pathogen transmission, even when the needle is changed, according to the CDC.
Some 200 of the more than 4,000 patients who were warned have signed up for free blood testing, WABC-TV reported.
The hospital seems to have changed its policy on the devices, though it was unclear when the change occurred.
"South Nassau has already implemented a hospital-wide policy that bans the use of insulin pens and permits only the use of single-patient-use vials to administer prescribed insulin treatments to patients," the statement said.
HIV can lead to AIDS, or the acquired immunodeficiency syndrome, and hepatitis refers to a group of viral infections that affect the liver, according to the CDC.