Health insurance stocks plummeted on Thursday after the nation's largest insurer slashed its profit forecast due to massive losses from Obamacare.

UnitedHealth Group, which warned that its earnings would take a $425 million hit, primarily due to worse than expected results from participating in the Obamacare insurance exchanges, saw its stock price fall $4.75, or more than 4 percent, to $112.46 in mid-morning trading.

Shares of Anthem, which has a larger footprint in the exchanges, saw its shares fall even further — down $9.41, or nearly 7 percent, to $127.89.

Among other major insurers, Aetna was down nearly 5 percent, Cigna was down over 4 percent, and Humana was down about 3 percent.