Distributions from HSAs are never deductible. Any distributions from an HSA that you indicate were used for qualified medical expenses are not included in taxable income to begin with. They don't become part of your AGI. In the HSA section you simply tell TurboTax how much of the HSA distribution reported on Form 1099-SA was used for qualified medical expenses.
If you itemize deductions, you'll need to enter in the medical-expense section under Deductions & Credits any medical expenses that you paid with distributions from the HSA. This is because TurboTax automatically subtracts the amount your HSA distributions applied to qualified medical expenses and only puts any remainder on Schedule A.
An HSA deduction only results from personal HSA contributions that you are eligible to make.