WASHINGTON — The Obama administration says more than 543,000 people signed up for coverage last week under the president’s health care law, as the 2016 enrollment season got underway. Premiums are going up for many plans, and consumers are being urged to shop around. The health care law offers subsidized private insurance for those who don’t have coverage through their jobs. The Department of Health and Human Services said Thursday that returning customers accounted for about two-thirds of those picking a plan for next year. The statistics cover 38 states served by the federal HealthCare.gov website. The administration is targeting five major metro areas for 2016 sign-ups: Dallas, Houston, northern New Jersey, Chicago, and Miami. The goal is to have 10 million people signed up and paying premiums.