I read this article yesterday, you might find it of interest.

https://www.forbes.com/sites/greatspeculations/2017/02/21/if-you-traded-bitcoin-you-should-report-capital-gains-to-the-irs/#44d521a9e3d8

Here is an excerpt:

The IRS considers cryptocurrencies, including Bitcoin, to be “intangible property.” Investors and traders holding cryptocurrency as a capital asset should use capital gain or loss tax treatment on sales and exchanges, with the realization method. For example, if you buy Bitcoins with U.S. dollars and later sell them for U.S. dollars, a capital gain or loss needs to be reported on that transaction.

Americans also trade Bitcoins or leveraged Bitcoin contracts on Bitcoin exchanges, and they should report realized capital gains and losses on each trade, even if the trader doesn’t convert underlying Bitcoin back into U.S. dollars.

It’s similar to having a foreign-based brokerage account, denominated in a foreign currency (i.e., Euros), where a trader buys and sells European equities held in Euros, and does not convert Euros back to U.S. dollars during the year. Two choices for tax reporting: Convert Bitcoin to U.S. dollars on each purchase and sale transaction using the Bitcoin market price that day denominated in U.S. dollars, or use Bitcoin as a functional currency, using an average Bitcoin vs. U.S. dollar conversion rate for the tax year.