The illness, in which the thyroid gland is attacked by the body's immune system, can cause a wide range of symptoms including chronic fatigue, depression, infertility, weight gain, and hair loss.
Healy was diagnosed with the condition in 2011 but is only now opening up about her health woes, revealing she fell pregnant with daughter Aoife days after discovering she has the illness - because she feared it would leave her unable to conceive.
She and her now-husband Ben Foden had been avoiding starting a family but the diagnosis made them both reconsider their plans for the future.
Irish singer Healy tells British magazine Fabulous, "I freaked out. Straight away I said to Ben: 'Let's stop not trying and see what happens.'... If it hadn't been for that check-up I'd probably have kept putting it off, thinking I had loads of time. And to this day I wouldn't be pregnant."