By Lucia Mutikani

WASHINGTON (Reuters) - The U.S. economy likely grew at a much faster pace in the second quarter than previously estimated, according to data on Thursday that showed a big jump in healthcare spending.

The Commerce Department's quarterly services survey, or QSS, showed healthcare outlays increased at a much brisker clip than the government had assumed in its last estimate of gross domestic product in late August.

As a result, economists said healthcare spending could add as much as three-tenths of a percentage point to second-quarter GDP growth, taking it to as high as a 4.7 percent annual rate.

"Second quarter growth is on track to be the strongest since early 2006," said Harm Bandholz, chief U.S. economist at UniCredit Research in New York.

Growth in the April-June quarter was previously reported to have increased at a 4.2 percent pace.

Prior to the QSS data, economists had already raised estimates for second-quarter growth by about 0.4 percentage points after construction spending and trade deficit figures for June came in better than the government had forecast.

But inventories at wholesalers and manufacturers were a bit weaker, suggesting restocking was not as strong as estimated in the GDP report last month. That saw economists shaving second-quarter GDP growth estimates by two-tenths of a percentage point to a 4.4 percent annual rate.

Growth estimates could shift again on Friday, with the release of retail data that may alter views on inventories and consumer spending.

The QSS, which provides a comprehensive count of service sector revenues, is frequently a source of GDP revisions.

With the introduction of President Barack Obama's signature healthcare law at the start of the year, healthcare spending has been particularly tough to estimate.

Jim O'Sullivan, chief economist at High Frequency Economics in Valhalla, New York, said the big difference has been in healthcare spending on hospitals.

In the last GDP report, hospital spending was reported to have increased at a 0.8 percent annual rate before adjusting for inflation, or to have declined at 1.4 percent rate when adjusted for inflation, said O'Sullivan.

"But the data that came out today shows a 10.6 percent annual rate in nominal terms, which is about a 8.4 percent rate in real (inflation-adjusted) terms," he said. "It will probably add another three-tenths or so to GDP growth."

Daniel Silver, an economist at JPMorgan in New York, said an upward revision to healthcare spending could lead to "a somewhat" stronger trend for services spending in the third quarter.

Early readings on consumer spending in the third quarter have been lagging other data such as manufacturing, housing and employment, that have shown the economy on a stronger growth path.

(Reporting by Lucia Mutikani; Editing by David Gregorio)