Government agencies are looking at two applications to sell and subsidise other brands of the thyroid drug levothyroxine after patient complaints about perceived side effects from GlaxoSmithKline's Eltroxin.

Abbot Laboratories has an application being evaluated, and a second company, Goldshield Healthcare NZ is expected to file an application by the end of the month.

Drug buying agency Pharmac, which oversees the drugs provided under Government subsidies, said it will consider over the next couple of weeks whether more than two subsidised forms of the drug were needed, and the likely costs involved.

At present some Eltroxin users have switched to unapproved alternative brands obtained through their doctors, which cost between $15 and $20 a month.

But experts at today's press conference said some of the flood of reported reactions may actually be psychological, rather than physiological.

GSK began supplying the new formulation in July last year, and until April and May of this year there were only about 30 reports of side effects filed to the Centre for Adverse Reactions Monitoring (Carm).

When numbers hit 43 in June, the reactions were publicised in advertisements, triggering extensive media publicity, and by today, Carm had reports of 810 reactions from the 70,000 patients taking the drug.

"That increases every time a news item runs on it," said Dr Stewart Jessamine, group manager of the Ministry of Health's drug regulatory arm Medsafe.

"There is some link between media coverage, and patients either identifying themselves as being potentially affected or coming to the realisation that they have been affected by the product, for whatever reason they care to give."

Dr Jessamine said Eltroxin had been used on cats and dogs after they had received radiation treatment.

"Anecdotally, we have had one or two people say their dog's never been the same since they changed to the new brand...but we shouldn't read too much into that," he said.

It was not known how the dog might communicate side effects such as headaches.

"These are routinely seen when patients swap brands of thyroxine but what is unusual about the New Zealand experience is that we are seeing so many reports," said Dr Jessamine.

Asked to what extent the surge of reports involved an element of "mass hysteria", Dr Jessamine said that in about half of the cases, symptoms could be explained by poorer absorption of the new formulation.

There were other side effects such as headaches, eye pain, conjunctivitis visual disturbance, and allergic reactions such as rashes and swelling of the skin, similar to hives occurring quickly after people swapped to the new formulation.

"To some extent, it's immaterial whether this is due to a physical cause...or whether its psychological," he said. "There are undoubtedly some people who do need an alternate brand."

A specialist endocrinologist, Dr Mike Croxan at the Auckland District Health Board, told the Wellington press conference that bioavailability was a problem in only few patients.

He had never seen a person with reactions of rashes and visual disturbance to thyroxine.

Sometimes there was not objective evidence for a reaction, but the description of "mass hysteria" was a bit strong.

"Some people do have symptoms when any medicine is changed," he said. "It may have a psychological origin: I think one has to agree that's a possibility in proportion of individuals."

The most important thing was that people switching brands or formulations, have their blood levels checked six weeks later for levels of thyroid stimulating hormone so the dosage rate can be adjusted.