NASHVILLE (AP) — A branch of the insurance company BlueCross BlueShield has split from Vanderbilt University Medical Center, forcing thousands of senior citizens to act quickly or lose medical coverage.

WSMV-TV reports (http://bit.ly/1XDQQnJ) that customers of BlueCross BlueShield's Medicare Advantage program recently received letters explaining changes to their network. The letters say Vanderbilt University Medical Center and its affiliated providers are leaving the network at the end of the year.

BlueCross BlueShield spokeswoman Mary Danielson says affected customers will need to change doctors or pay higher, out-of-network rates to stay with their existing treatment plans.

Vanderbilt spokesman John Howser tells the news agency that the hospital and the insurance company could not reach an agreement. He says the hospital has set up a website to answer questions, vanderbiltmatterstome.com.