The amount could have come from an amount transferred from your W-2, a 1099, or a 1095-A. Medical expenses amounts from those forms get automatically transferred to the medical expenses worksheet. If you are using the Desktop program, you can click on Forms and review the Medical Expenses worksheet.
If you are using TurboTax Online, you can go through the Medical Expenses interview. You can either search for Schedule A and use the Jump to link, or you can go to the Medical section from the Deductions & Credits menu.