If the check from the insurance company was for 2018 premiums, then you are correct. You would not include the amount as income, but rather, you would reduce the amount of deductible medical expenses by the amount of the reimbursement.
If the check was for a prior year's expenses, you would need to see if you itemized deductions in that year, and if you did, see if you used medical expenses on your itemized deductions. If your expenses were enough to contribute to itemized deductions, then the reimbursement you received would be included as taxable income as a taxable reimbursement of an itemized deduction (similar to a state tax refund if you itemized deductions and used the state taxes withheld as part of the deduction: state tax refund in the following year is taxable.)