Transcript for Mariah Carey speaks out about her struggles with bipolar disorder
Reporter: "Emotion." ???Tion ??? Reporter: "We belong gather." ??? We belong together ??? Reporter: The single biggest Christmas song of alme. ??? All I want for christmas U ??? Reporter: That incredible vocal rang??? and glamorous exterior, sst Mariah careyling tod a private struggle,elling "People" magazine about her 17-year battle with bipolardisorder saying, until recently I liven denial and isolation and in constant fear someowould expose me. It was too heavy a burden to ca and I simply couldn't do thatymore." She was diagnosed W bipolar disorder backn2001. And did not want to believe it. She believed it wasething that could really damage her career. And so she didn't get treatment for . And I think that we saw the facts of that over theyears. Reporter: Almost 6 million americuffer F bipolar disorder. Symptoms like extreme highs a lows, and prolonged sleeplessness. Dr. Robi Ludwig is a psychoist. She has not treated Mariah has workedh other patients with bipola disorder. Is it surprising to see a celebrity like Mariah Carey be diagnosed with BIR? I'm not really shockedhen I see anyone died with a mental health issue. It's probably more common to have aental health issue not. Tell us E what bipolar 2 disorder is. Bipolar disorder is a mood disordh high highs and very allows. Bipoladisorder like Mariah will hav hypermania. Ey might feel grandiose, the might have feel hypeua ight have poor judgment, they might be irrita Repor Carey postin'm grateful to be sharing this part of my life with you. She described periodsf loneliness, sadness, she couldnsleep, she was up working. She'd wond work and work and work until she hit a wall. Reporter: Online tal wave of sort. Fans responding, thank you for being grave and a voice Al illness, we got your back. On Twitter the national alle on mental illness tweeting, you arot alone, you are inspiring many by sharing your experienceith bipolar disorder, thank Y for being stigma-free. Carey says sheea her struggle with bipola disorder the same yeah released "Glitter" featuring singles like "Lover boy." ??????That mere, 2001, markedange occurrences in herife cluding this bizarremp appearce on mtv's total request live." Mariah Carey is stripping. Reporter: One week later sh was hospital for physical and mental exhaustion.should these have been red fl You can seeanic or odd behaviort may not be due to havingbipolar illness, it be due T something else. Reporter: Despite being diagnosed 17 years ag Carey telling "Pe" S didn't want to believe it. She she sought Trent after having what she Cal the hardest couple of years that she'd ever gone thh. We saw some of thosers play out in public. There was a brief engagement. E was doing a reality show on television where everyone W like, why is she D THA reality show? I can certainly see W it would be sca for any celebrity to reveal thearker sid or the less-beautiful side of L that they dealingwith. Because of fear that they will lose fans or might notas vied for. Reporter: During her car she has B open about her Pers life. Here backstage with Michael Strahan on the set Ofer V residency in2015. It's like a clean slate. It's notust a show, it's the Al number one to infinity. It's the show,r one to infinty. 'S the place I'm at in my life, which brand-new moment for Reporte invitingarbara Walters into her home with family in 2013. Casays she is now in therapy and takingmedication. What would M someone who receives that kind ofosis delay treatment? When someone has bipolar disordry often they feel good. So it could bhatiah was feeling good more ofthan she was feeling bad. And that she was just indenial, couldn't belie that she had this diagno which is not S common. Reporter: Recently, other ceities have gone public about their struggles with ntal health, like to. I actually have bipolar disorder. Mental th affects so manypeople. And we need T T the stigma away from it. Wn a celebrity out and talks about anything that is a personal struggle is going toel other peo going through the same thg.it's going to lift the stigma. It's going take it okay to talk a it. Whethes princess Diana talking abouing disorderether it'sty Ford talking about Alli these are Al ry powerful messages to ople. Reporter: Careyelling "People" also hopes her latin Wil hope remove the stigma from her condition polly giving hope to pe who feel alone. I H M people with polar disorder in my practice. They live highly successful S, both personally and professionally. There's Eason why they a disastrous life or hopelessness at all. Mariah Carey is one oe biggest music stars on the planet. To hear someone L that whose work and voice and talent is in ou DNA talk about this vulnerability is an incredibly powerful message that will give people a lot of hope. Rr: For "Nightline," I'm Ariel rechef in New York
This transcript has been automatically generated and may not be 100% accurate.