The TV star, 27, shared a video of herself being treated with red light therapy for alopecia - which is a general term for hair loss - on TikTok.
Although Look Both Ways star Reinhart did not share many details about the diagnosis, she wrote on the clip: "Was just diagnosed with alopecia in the midst of a major depressive episode."
In the footage shared to social media, she can also be seen lip-syncing to audio which said: "I'm pushed beyond the limits of what a person should be pushed to endure."
The Hustlers actress also captioned the post: "Red light therapy is my new best friend", using the hashtags alopecia and mental health.
It is not always clear why alopecia develops, but experts believe a number of factors may be involved - including an autoimmune response.
This is where the immune system mistakenly attacks healthy cells, such as the hair follicles, from triggers such as stress, trauma or illness. Hair loss may also be linked with nutritional deficiencies or certain medications.
Reinhart, known for playing Betty Cooper in teen drama series Riverdale, has previously spoken about having depression and anxiety, and the importance of openly discussing mental health.
In 2019, she told Glamour UK: "I have spells of time where I feel completely unmotivated, I don't want to do anything and I question myself.
"I don't know how to handle stress very well. I find that talking about it and sharing my experience with other people, and reminding myself that I'm not alone, has been incredibly therapeutic."
19°C