The Irish singer was diagnosed with Hashimoto’s Disease, a condition that affects the thyroid gland, back when she was touring with the girl band.
The disorder altered her voice, and she now has to take daily medication to treat it “forever.”
Una opened up about her diagnosis during a chat on Instagram Live with Canadian singer-songwriter Emm Gryner yesterday.
She said: “I remember when I joined The Saturdays first - you probably hear it a lot on the early recordings, my voice was a lot huskier.
“Now I do have a husky tone that comes out sometimes, like a rasp... It just comes out sometimes, I can’t put it on, it actually hurts to even do that.”
“I found that it was happening more and more, and I went in one day to demo a song we were doing with the girls and the producer went ‘Okay, can you do that again?’ and he looked really worried.”
“And he said to me ‘You have a very unusual voice’ because on that day it was so husky, and I think a lot of people liked the huskiness but it was getting to the point where it was just extra husky all the time and I knew there was something wrong.
“The doctor came in to see all of us and he kind of looked around my neck, and he was like ‘you’ve a bit of swelling around your throat there. Have you ever had your thyroid examined?’
“And I said no, so I had my blood checked to check how my thyroid levels were and it turned out I’ve an underactive thyroid, so I’m hypothyroid.”
Hypothyroidism can cause notable voice changes, such as low voice, roughness, reduced range, and vocal fatigue and if left untreated, can cause a number of health problems.
“I am now on thyroxine, I have to take a thyroxine tablet daily, forever,” Una explained.
“But I am totally healthy and completely normal, but that was the side effect of the early days of it.
“One of the side effects of having an underactive thyroid is hoarseness. I’m lucky that I caught it on time when I did... I’m perfectly fine now.
“There are a lot of different side effects to an underactive thyroid, and I didn’t have any of the big obvious ones apart from that my neck was slightly swelling and I can see in old pictures now where that was.”
Una added that her diagnosis was initially “shocking” but said that it’s important for people to get checked.