The dollar hit a two-year high on Tuesday as slowing global growth and more aggressive monetary easing overseas sent investors into the reserve currency.

The U.S. Dollar index rose to 99.37, its highest level since May 2017, when it hit 99.696. The index, which measures the greenback against a basket of other currencies, is teetering near the 100 level, which it hasn't hit since April 2017.

"Going forward, the dollar will have a hard time declining unless the Fed gets aggressively dovish, simply because global growth remains lackluster and global central banks are easing policy or are about to ease policy," said Tom Essaye, founder of the Sevens Report.

Although more than $15 trillion of government bonds are trading at negative yields around the world, U.S. bonds are still in positive territory. Growth is slowing at a much faster rate abroad and global central banks are easing at unprecedented levels. Although the Federal Reserve cut interest rates in July for the first time since the financial crisis, the U.S. economy is strong compared with the rest of the world. And it's unclear whether the Fed will keep cutting rates regularly. Fed policymakers meet in two weeks.