The Spanish dancer’s fans were concerned after he revealed that he had spent the night in hospital this week.
He has now explained that he was suffering from the condition rhabdomyolysis.
"Thank you to everyone for the lovely messages," Marquez wrote on his Instagram Story.
"Just to clarify, I didn’t have COVID or nothing related to COVID vaccinations."
"I had rhabdomyolysis," he explained, saying that it was a "condition in which damaged skeletal muscle breaks down rapidly which leads to my CK enzymes levels to be very high and that could lead to my kidneys stop (sic) working".
Earlier, Marquez had shared a string of selfies showing him in a hospital bed.
The star – who is engaged to actor Gemma Atkinson – told fans: “I was looking forward for Christmas at home but that was interesting 24h in hospital.
"@glouiseatkinson thinks it was an excuse to avoid all the wrapping of the Christmas presents…
"Anyway… everything is okay and I am back home having cookies and cuddles of my little nurse."
He went on: “Thanks to the Doctors and Nurses at Fairfield Hospital for being so lovely to me.
According to WebMD, rhabdomyolysis is a “serious syndrome due to a direct or indirect muscle injury”.
“It results from the death of muscle fibres and release of their contents into the bloodstream. This can lead to serious complications such as renal failure.”
Watch: Gemma Atkinson and Gorka Marquez got engaged on Valentine's Day