"Nicole has decided to undergo diagnostic treatment to determine why she's not been putting on any weight.
"She is working with a team of doctors and specialists whose focus is nutrition. It is important to Nicole that she achieves this goal in a healthy way as this is not a treatment for an eating disorder."
A source close to Nicole, 25, told People magazine: "She's tired of everyone saying you don't eat, because she does.
"She wants to gain weight. It wasn't anyone saying you have to go do this - it wasn't an intervention. It was her saying I'm tired of people saying this about me, I'm going to go get some tests. It was a personal decision."
Earlier this year, Nicole admitted she had problems with her weight but partly blamed it on her split from on-off boyfriend, Adam 'DJ AM' Goldstein.
She said: "I know I'm too thin right now. I'm not happy