The mother-of-five uploaded a picture to Instagram on Monday afternoon (23 November) showing her eldest child wearing a hospital gown while standing in a hospital ward.
Her caption read: "My brave boy."
Another photo of Katie and Harvey together carried the message: "Love him so much."
Katie didn't reveal why Harvey, 13, was in hospital but fans sent their well wishes and love to the mother and son via the comments section of Instagram.
Harvey has been in hospital many times over the years because of a series of conditions he was born with, including visual impairment, autism and Prader-Willi Syndrome.
Katie's often spoken publicly about life with Harvey and lends her support to several children's charities. Harvey has also appeared on TV with his mum, most memorably on ITV's This Morning in March, when he melted the nation's hearts by calling his mum "babe".
During that interview, Katie had explained Harvey takes medication five to six times a day because his life depends on it, as well as a hormone injection every night. The whole family's schedules have to be in sync so as to keep Harvey's routine the same every day, meaning they all attend appointments about his medical treatment and education.
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