T-Mobile's big gains in the marketplace aren't coming cheap.

The fourth-largest U.S. wireless carrier capped 2013 with a big increase in subscribers and projected more gains this year, but the cost of winning that business hurt its margins and widened its loss in last year's final quarter.

The results show how T-Mobile US Inc. has managed to shake up the wireless market by doing away with carrier contracts and splurging on marketing. But they also raise questions about how long the company can keep up the costly pace.

T-Mobile shares fell 5.7% Tuesday, after the company posted a quarterly loss of $20 million, compared with a year-earlier loss of $8 million. Revenue rose 39% to $6.8 billion, reflecting the merger with smaller rival MetroPCS Communications Inc.

The carrier, which had long leaked customers to bigger rivals Verizon Wireless, AT&T Inc. and Sprint Corp., added 869,000 of the industry's most valuable "postpaid" customers in the fourth quarter, bringing total additions for the year to two million. T-Mobile said in 2014 it would add two million to three million more postpaid customers, those better-off subscribers who pay at the end of the month and tend to switch carriers much less frequently than customers who pay as they go.

As of the end of the fourth quarter, the company had 46.7 million customers.

The cost of acquiring new customers jumped 40%. T-Mobile's gains have eaten into competitors' subscriber additions and have sparked concerns that a price war may break out in an industry that has been logging steady gains in the amount of revenue paid by subscribers.

The carrier's executives, however, signaled strongly that they don't intend to start one, particularly given the heavy cost of rolling out new, faster network technology called LTE.

"I don't think it's an environment that would result in an all-out price war," T-Mobile finance chief Braxton Carter said in an interview. "There wouldn't be adequate capital to support the buildout."

T-Mobile has done away with industry features like contracts and international data fees that have long annoyed customers, but it hasn't actually lowered prices much, Mr. Carter said.

Its average service revenue per customer has fallen, but that is largely because the figure no longer reflects device subsidies, which were replaced by smartphone payment plans that are accounted for in equipment revenue. "There really hasn't been a step function down in pricing," he said. "What's really happening is that total consideration--total payments received from the customer--actually was up."

AT&T, Verizon Communications Inc.'s Verizon Wireless and Sprint have followed T-Mobile's moves by cutting prices selectively on some group plans and data buckets, but haven't implemented across-the-board cuts.

Antitrust authorities are pleased with the effect T-Mobile is having in the market following the 2011 rejection of AT&T's $39 billion deal to buy the company. Sprint is now weighing whether to make a bid of its own, a deal regulators have said they would view with skepticism, according to people familiar with the matter.

T-Mobile's executives declined to talk about possible mergers Tuesday.

The company merged into MetroPCS in May, gaining a public stock listing. It was previously operated by Deutsche Telekom AG, which still holds about two-thirds of the company's stock.

Write to Everdeen Mason at everdeen.mason@wsj.com and Andrew Dowell at andrew.dowell@wsj.com

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