Lo Bosworth says a battle with depression led to her "worst year ever" in 2016.
"I started feeling very off in the fall of '15. Racing mind, impulsive behavior, insomnia," Bosworth wrote.
"2016 arrives, and so does the therapy and the medication. I feel okay for moments of time, and then plunge back into the darkness again and again."
"Up and down, up and down. Tears, moments of extreme highs, confusion about why you don't feel completely back on your feet after the standard 4-6 weeks of allowing the meds to 'kick in,'" she said. "I wouldn't wish my 2016 on my worst enemy."
Bosworth said doctors were confused by her lack of progress until a blood test in September showed she had "severe deficiencies" of Vitamin B12 and Vitamin D.She later discovered the deficiencies were linked to mutations on her MTHFR and MAOA genes.
"You can develop, amongst other things, depression, anxiety, fatigue, headaches, dizziness, and a lot of other vague and uncomfortable symptoms," the star explained.
"[I will now take] the highest quality, most bio-available methylated form of [Vitamins B12 and D] every single day for the rest of my life," she said.
"I'm living proof that you should fight, you should explore, you should take your health into your own hands and be your own advocate, and that you CAN come out the other side."
Bosworth came to fame on the MTV series "Laguna Beach" before co-starring on "The Hills."
She launched her own feminine hygiene care line, Love Wellness, in October, which includes a cleanser, a moisturizer and supplements.
