Speaking to Steph McGovern, he said his wife Anne had become concerned when he couldn’t remember information about his own album on The One Show.
After growing fears, he underwent a scan, which concluded there was “no loss of brain mass and very few lesions”.
He later took a blood test which determined the cause.
He divulged: “What I’m suffering with is a swollen tongue, which causes you to slur your words.
“It slows down, not your thoughts, your thoughts are there but transferring those thoughts to the mouth, to talk coherently.”
According to the NHS, an underactive thyroid (hypothyroidism) is usually treated by taking daily hormone replacement tablets called levothyroxine.
Levothyroxine replaces the thyroxine hormone, which your thyroid does not make enough of.