The nation’s largest medical laboratory company provided possibly erroneous results to thousands of people who had their vitamin D levels tested in the last two years, the company has acknowledged.

The company, Quest Diagnostics, has already sent letters to thousands of doctors listing the patients who might have received “questionable” test results and is offering free retests. The company said it had fixed the problems.

An erroneously high result may mean patients will not take vitamin D supplements when perhaps they should, doctors said. And an erroneously low test result might lead in rare instances to a toxic overdose of vitamin D. When the Quest tests have been inaccurate, the reading has typically been too high, although not in all cases.

Quest’s action represents “the largest patient test recall I’m aware of in my 20 years in the business,” said Robert Michel, editor of The Dark Report, a newsletter for pathologists that first reported on Quest’s action.

The incident could raise calls for more regulation of diagnostic testing at a time when diagnostics are playing an increasingly crucial role in guiding medical treatment. Many laboratory tests, including Quest’s vitamin D test, do not require approval from the Food and Drug Administration.