NEWINGTON, N.H. — Gym company Planet Fitness has started its initial public offering. The Newington, New Hampshire-based workout center chain announced 13,500,000 shares of its Class A common stock Monday. The company says underwriters will have a 30-day option to buy up to an additional 2,025,000 shares. The price range for the IPO is estimated to be between $14 and $16 per share. As it prepared to go public, Planet Fitness sought a change in state tax laws that would’ve kept it from paying higher business profits taxes.