* Q3 loss C$0.25 per share vs loss C$0.71 yr-ago

* Loss driven by changes to actuarial assumptions

* Says 2010 income may be 33 pct lower than pre-crisis

* Maintains dividend (Adds details, outlook for normalized earnings, byline)

By Andrea Hopkins

TORONTO, Nov 5 (Reuters) - Sun Life Financial Inc (SLF.TO: Quote, Profile, Research) reported a net loss on Thursday due mostly to a change to the assumptions it uses to value its books, but cited underlying strength as it maintained its dividend payout.

Canada's No. 3 life insurer also said it believed 2010 earnings could be as much as 33 percent below what they were before the financial crisis sideswiped the industry, offering a much-sought-after view of what normalized income might be.

Sun Life said it had a net loss of C$140 million ($132 million), or 25 Canadian cents a share, for the third quarter. That compares with a net loss of C$396 million, or 71 Canadian cents a share, for the same quarter a year earlier.

The Toronto-based insurer said in August it would update its equity and interest rate-related actuarial assumptions used to value its variable annuities, segregated fund and some fixed annuities, so the one-time negative impact was expected.