The IRS ruled that bitcoins and other cryptocurrencies are property, not currency. As such, you determine gain or loss in the same was as any other type of property. Compare what you paid for the bitcoins and what you received on the sale (making adjustments for commissions, etc.).
Items, such as interest, state and local income and sales taxes, charitable contributions, and medical deductions, claimed on Schedule A of Form 1040. Itemized deductions are subtracted from adjusted gross income to arrive at taxable income. The amount of itemized deductions is also subject to a reduction when adjusted gross income exceeds certain limits.