New figures from the Ministry of Health show a surge of adverse reactions to the only thyroxine medication licenced in New Zealand.
The figures released by the Green Party show that this year 571 people have reported adverse reactions to Eltroxin, which is used to treat the metabolic disorder hypothyroidism.
The Greens say the number of complaints means the Government has no option but to make a subsidised alternative available.
However, medicine safety regulator Medsafe says independent testing has found Eltroxin to be safe.
A Te Awamutu woman whose 80-year-old mother was hospitalised after her thyroid medication was changed is calling for urgent action.
Angela Cole's mother has been treating an under-active thyroid with Eltroxin for the past 20 years. She says her mother has never experienced any side-effects from the medication but blames it for her admission to hospital earlier this year.
Ms Cole says her mother spent more than four months in hospital, before switching to an alternative, non-subsidised thyroid medication.