While the overhaul gave insurers more customers through a coverage expansion that started this year, it also heaped costs on their balance sheets, including an industry-wide tax that is non-deductible. UnitedHealth said its provision for income taxes in the second quarter jumped 25 percent to $989 million.
The insurer's largest operating expense, medical costs, also climbed 6 percent to $23.52 billion.
Overall, the insurer earned $1.41 billion, or $1.42 per share, in the quarter. That's down from $1.44 billion, or $1.40 per share, in the 2013 quarter, when it had more shares outstanding.
Revenue rose 7 percent to $32.57 billion.
Analysts expected, on average, earnings of $1.26 per share on $31.9 billion in revenue, according to FactSet.
Health insurance is UnitedHealth's largest business, but it also has seen growth from its Optum segment, which sells information technology services and perform pharmacy benefits management. Optum's operating earnings jumped 23 percent to $728 million in the quarter.
UnitedHealth also cited the health care law as a chief reason its first-quarter profit slid 8 percent. The overhaul-related hits shouldn't surprise investors. The insurer said back in December that it expected the federal law to reduce its after-tax operating earnings by $1.1 billion in 2014.
But the insurer has still earned enough so far this year to hike the quarterly cash dividend it gives shareholders by more than 30 percent to 37.5 cents per share. The new payout, announced in June, now triples the dividend the insurer debuted in 2010.
UnitedHealth now expects 2014 earnings to range between $5.50 and $5.60 per share. That compares to its previous forecast for $5.40 to $5.60 per share. It also raised its revenue estimate to $130 billion from $128 billion to $129 billion.
Analysts predict, on average, earnings of $5.52 per share on $128.38 billion in revenue.
UnitedHealth Group Inc. is the first insurer to report earnings every quarter. Many see the insurer, a component of the Dow Jones industrial average, as a bellwether for other insurers.
Company shares rose $2.13, or 2.5 percent, to $85.89 in morning trading Thursday. The stock was up more than 11 percent for 2014 through Wednesday's close.