Shifting conditions in the commercial health insurance marketplace and underperformance in the public health insurance exchanges caused Aetna Inc.'s total membership and net income to decline in the third quarter of 2015.

Aetna's third-quarter profits fell 5.8% to $560.1 million compared with results from a year ago, due primarily to a smaller and more costly pool of enrollees in insurance plans purchased through public exchanges than the company had anticipated.

The Hartford, Connecticut-based health insurer reported total revenues of $14.95 billion for the three months ending Sept. 30, a 1.5% increase over third-quarter revenues collected in 2014 driven largely by membership gains in Medicare and Medicaid accounts.

Senior executives at Aetna told investment analysts during a conference call on Thursday that the company will pull out of public exchanges in Utah and Kansas and adjust its plan offerings in the 15 other states where it will continue to participate in the exchanges.

“We view (the public exchanges) as a long-term market potential,” Mark Bertolini, Aetna's chairman and CEO, said during the conference call. “We like the dynamics of the markets we're in, and it's been our view that we would go into markets where we thought we had a cost structure that could develop an affordable product for people in those communities. Our changes for this year are related to our ability to do that.”

Earlier this year, Aetna announced that it was also ending its participation in the District of Columbia's public exchange in 2016, but adding Kentucky to the states where it offers exchange-based plans.

“We think it's way too early to call it quits on the (Affordable Care Act) exchanges,” Mr. Bertolini said. “We still view them as a big opportunity for the company.”

Commercial membership falls

Aetna's total commercial membership — which includes both individual and employer-based enrollments — continued to slip in the third quarter of 2015, shedding nearly 400,000 enrollees since Sept. 30 of last year.

Karen Rohan, Aetna's president, said during Thursday's conference call that much of the membership attrition in the past year has been due to several of the company's large employer clients electing to offer their employees coverage from multiple insurers — referred to in the health insurance industry as “slice business” — often through a private health insurance exchange.

“We had some jumbo accounts that decided to offer their employees more choice by going into slice, and we had some terminations,” Ms. Rohan said.