T-Mobile US Inc. posted double-digit top- and bottom-line growth in the final quarter of the year, as it continues to snap up customers and pressure its competitors.

Results beat Wall Street expectations and shares, which have surged 73% over the past 12 months, added another 1.8% premarket to $61.99.

The wireless carrier has been grabbing market share recently and during the fourth quarter extended its streak of adding more than 1 million customers to 15 consecutive periods. The company added 2.1 million net customers in the latest quarter.

As T-Mobile's customer base expanded, service revenue grew 11% to $7.25 billion. The company added 1.2 million branded postpaid customers -- the most lucrative type of wireless subscriber -- in the quarter.

"That's three years in a row that we've added more than 8 million customers and taken all of the postpaid phone growth in the industry," Chief Executive John Legere said in prepared remarks.

For 2017, T-Mobile forecast 2.4 to 3.4 million branded postpaid net additions, below the 4.1 million added in 2016.

But T-Mobile is aiming for higher profitability, targeting adjusted Ebitda -- or earnings before interest, taxes, depreciation and amortization -- of $10.4 billion to $10.8 billion, compared with $10.4 billion last year.

In all for the December period, T-Mobile earned $390 million, or 45 cents a share, up from $297 million, or 34 cents a share, a year earlier. Analysts polled by Thomson Reuters were expecting 30 cents.

Revenue climbed 23% to $10.18 billion, topping analysts' expectations for $9.84 billion.

In October, T-Mobile reached a settlement with the Federal Communications Commission valued at $48 million for failing to adequately inform consumers of wireless data restrictions on plans it called "unlimited." The FCC said the carrier applied slower data speeds once certain usage thresholds were hit -- without telling customers.

Write to Anne Steele at Anne.Steele@wsj.com