The fortification of more staple foods with iodine should be considered to ensure children are getting enough of the essential nutrient, Otago University researchers say.
They made the suggestion after a study of school children found some children may still have enlarged thyroids because they were not getting enough iodine.
Since 2009 it has been mandatory to use iodised salt in commercial bread, and the researchers from the university's department of human nutrition said that had made a positive impact.
For their study they measured biochemical markers of iodine status in 147 primary students aged eight to 10 from 18, randomly selected schools in Dunedin and Wellington.
They found the children's median level of a short term marker called urinary iodine concentration (UIC) was almost double that found in a 2002 study, although it was still just within the range for adequate iodine status.
The median level for the concentration of the protein thyroglobulin (Tg) in the children's blood was still within a range indicating mild iodine deficiency, the researchers said.
Tg was produced by the thyroid gland and has been proposed as a longer term marker for iodine status.
Study lead author Dr Sheila Skeaff said the raised Tg levels suggested some New Zealand children may still have enlarged thyroids because they were not getting enough iodine.
Earlier research found that correcting mild iodine deficiency led to small but significant improvements in children's performance in cognitive tests.
Measures parents could take to increase their children's iodine intake included always using iodised salt, while dairy products, eggs, fish and other seafood were also good sources of iodine, Skeaff said.
It was important that children who ate little or no commercial bread products, which were now the main source of dietary iodine, received a sufficient intake from elsewhere.
"Our results also suggest that fortifying other staple foods needs to be considered if we want to ensure that New Zealand children have good iodine status," she said.
The study, published in the British Journal of Nutrition, said more research was needed to assess the impact of fortification on the iodine status of other groups, such as pregnant women.
When mandatory fortification was first explored in this country, other foods had been considered including sweet biscuits and breakfast cereals.
Only bread had been chosen because of concerns some people could get too much iodine.
Iodine deficiency had been common in many parts of the world for thousands of years, the study said.
Iodised salt had been introduced in the first half of last century, and in many countries where it was used regularly, iodine deficiency and goitre - enlargement of the thyroid gland - had all but disappeared.
A change in food habits had led to a re-emergence of iodine deficiency in many parts of the world including Britain, Australia and New Zealand.
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