Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of weight-loss specialist Weight Watchers International (NYSE: WTW) soared a staggering 43% in early Thursday trading after both its quarterly results and full-year forecast blew out Wall Street expectations.

So what: Fueled by strong demand for its subscription-based products, Weight Watchers' fourth-quarter profit jumped 160% to $0.66 per share, while analysts were anticipating $0.56. Rising costs have weighed on the company's results in previous quarters, but the recent launch of its new PointsPlus weight-loss program, as well as drastically improved Web-based sales, seems to be turning things around.

Now what: Those tailwinds are likely to keep blowing in the company's favor. Thanks to booming enrollment in the U.S. and U.K., management now sees 2011 earnings of $3.50-$3.85 per share, versus the average analyst estimate of just $2.77 per share. The shares might be hitting all-time highs today, but with a reasonable 16-ish forward P/E, Weight Watchers might still have plenty of room to run.

Interested in more info on Weight Watchers? Add it to your watchlist.