Kim Kardashian West says Caitlyn Jenner has shown her how to live "an authentic life."

Caitlyn Jenner

Caitlyn Jenner

The 35-year-old reality TV star posted a gushing tribute to the former Olympian, who was known as Bruce Jenner before undergoing gender transition in May, on Instagram in honour of her 66th birthday on Wednesday (28.10.15).

Alongside of a photograph of them enjoying a pre-birthday lunch, the pregnant 'Keeping Up with the Kardashians' star wrote: "Happy Birthday Caitlyn! Are you 66 or is Caitlyn 1? lol That was the debate at our bday lunch yesterday! Have the best day ever filled with lots of love & laughs! You have shown me what it's truly like to live an authentic life without caring what the world would think! I love you!"

Kim also dedicated another message to the 'I am Cait' star, who split from her mother Kris Jenner in 2013, on her website, writing: "What a brave soul! I'm happy you are living your authentic life! You are my Woman Crush Wednesday. Happy birthday!!!"

Her younger sister Khloé, 31, also posted a message on her own website, writing: "Happy birthday to one of the most inspirational people I know. It's your first birthday, and I couldn't be more thrilled for you to celebrate it style. Get glam, girl! (sic)"

Caitlyn thanked her fans for sending her their well wishes via social media by sharing a video montage of "66 special moments from this incredible year" via Flipgram.