"I've always known my little bro was tough—but this tour showed me a whole new level of strength," he wrote in an Instagram post alongside a photo of him with Justin, Lance Bass and JC Chasez.
Lance also posted a message of support on his Instagram Story, writing, "Proud of you brother. Wishing you some well-deserved rest and so much love."
Justin shared the diagnosis with his fans in an Instagram post Thursday that reflected on the end of his two-year Forget Tomorrow World Tour.