I received a reimbursement from my employer for my healthcare premium in the form of a check, is this considered taxable supplemental wages?

If an employee pays the premiums on personally owned health insurance or incurs medical costs and is reimbursed by the employer, the reimbursement generally is excluded from the employee’s gross income and not taxed under both federal and state tax law. This includes premiums for tax dependents and opposite-sex spouses. However, there are some circumstances in which the reimbursement is taxable income, including the following:

• If an employer simply pays the employee an extra amount and does not specify in writing that the amount must be used to pay the health coverage premium, it will be taxable to the employee as income.
• If the employer is self-employed, any reimbursements for their own or their dependents’ health care costs are taxable income to the self-employed employer.

http://healthcoverageguide.org/reference-guide/laws-and-rights/tax-implications/