Cassie was diagnosed with a postpartum thyroid disorder following the birth of her daughter.
The 33-year-old singer welcomed daughter Frankie Stone with her husband Alex Fine in December, and has said that after battling with her post-baby weight, she eventually got a diagnosis from a doctor who told her an underlying complication with her thyroid was to blame for her inability to shed the pounds.
In a blog post titled 'What Healthy Looks Like to Me', she wrote: "I lost 20lbs within the first couple of weeks which I was told was normal and then another 10 gradually over the next 2-3 weeks.
"I started to workout when I had a goal of 30lbs to go and when my doctor cleared me to. I went pretty hard at first. By mid March I was down another 15 and feeling pretty good. I had a bit of unexplained anxiety, but I was able to work through it and down another 15 meant I only had 15lbs to go to get back to my pre-baby weight. (sic)"
Cassie says her weight loss only lasted "a couple of weeks" before she gained 15lbs back again, which prompted her to see a doctor.
Explaining her diagnosis, she said: "Basically my thyroid was hyper for a short period (hence the anxiety and weight loss) and then slowed down causing me to gain weight back. From what my doctor told me it's not super common. About 5-10 percent of women experience it within the first year after childbirth. (sic)"
The 'Official Girl' singer was worried her diagnosis would develop into Hashimoto's disease - which is an autoimmune disorder that causes an underactive thyroid - and so overhauled her lifestyle to change her diet and exercise regime.
She said: "The next 3 months following were confusing and depressing to say the least. I changed my diet paying attention to things that I reacted negatively to (ie, bloating). I actually started roughly following an autoimmune diet (AIP) in fear that my thyroiditis would turn into Hashimoto's Disease (an autoimmune disorder).
"I had completely lost my mind, but I'm glad that I did. I needed to understand what was happening with my body.
"I went from creating and bringing life into this world to trying whatever I could to get back to feeling like myself again. It wasn't about my weight anymore, it was about being happy, feeling good and not developing a disorder I've never heard of. (sic)"
Now, Cassie doesn't put too much of a focus on her weight loss goals, and is learning to be happy as long as she's "taking care" of herself.
She concluded her post: "Today I'm in better shape than I was before the baby. I honestly don't think I've ever been in this good of shape. Working out has given me a place to get rid of any negative energy and emotions that I might have, boost my serotonin and practice discipline. Every day that I workout isn't perfect, but I show up and I push myself."
"Whatever your goals are, choose to be healthy. For me, this is what it looks like when I'm taking care of myself. This is what healthy looks like for me."