Also in "The heat index," Molly Sims opening up about dropping her baby weight. She found out that something was working against her. Abbie Boudreau has the story. Reporter: Model and actress -- I'm a... See More
Also in "The heat index," Molly Sims opening up about dropping her baby weight. She found out that something was working against her. Abbie Boudreau has the story. Reporter: Model and actress -- I'm a bored housewife. Reporter: Molly Sims, known for her bombshell looks, especially in lingerie. But after giving birth to her baby boy and gaining 72 pounds, the supermodel, turned supermom, wondered if she would ever get that killer body back. I stepped on a scale. Actually have a picture of my scale and my feet. And I was 204. Reporter: Molly says six weeks after her c-section, she hit the gym. I would work out for 1:45, 1:50. Reporter: You were working out. I wore a trash bag. To sweat more. Reporter: Like, you were determined. Turn on the heat. I had a corset on. Reporter: She tried acupuncture. But that baby weight wasn't coming off. My neck looked like a football player. And I kept asking people, I don't feel right. Something's wrong with me. Oh, it's your hormones. Don't worry about it. You're in Hollywood and you're putting so much pressure on yourself to lose weight. No, something is wrong. Reporter: Turns out, Molly's intuition was right. Four months after giving birth, her doctor took one look and knew what was wrong. He's like, something is wrong with your thyroid. My neck was huge. Reporter: Dr. Rochelle tabib says thyroid problems can be triggered during pregnancy and can lead to problems if left untreated. It can affect your mood, the way you're interacting with your family, your weight, your me top limit. Reporter: Molly says she was diagnosed with thyroiditis. She finally lost the weight. My neck slowly started going down. It took about six months, eight months. It took a year. Reporter: And it was all worth it. In fact, Molly says she and husband are trying for baby number two. For "Good morning America," Abbie Boudreau, ABC news, los Angeles.
This transcript has been automatically generated and may not be 100% accurate.