To the Editor: Iodine is essential for production of thyroid hormone. The recommended daily intake is 100 μg for children, 150 μg for adults and 250 μg for pregnant and lactating women.1 Sporadic surveys of population iodine intake in Sydney, New South Wales, between 1985 and 1992 showed median levels of urinary iodine excretion (UIE) > 200 μg/L, indicating iodine sufficiency.2 However, a recent national study demonstrated mild iodine deficiency (median UIE < 100 μg/L) in New South Wales and Victoria, borderline levels in South Australia and adequate intake in Queensland and Western Australia.3

The major sources of dietary iodine are dairy milk and dairy products, seafood and iodised salt. In Australia, few people purchase iodised salt, and, except in Tasmania, the food industry does not use iodised salt in the production and preparation of food.4 For decades, milk contaminated with iodine residues from sanitising solutions (iodophors) used in the dairy industry has probably been the largest source of iodine in the Australian diet.

We undertook a survey of the iodine content of milk samples from supermarkets around metropolitan Sydney in 2001 and 2004. In each year, iodine levels were measured in 13 samples, comprising a range of milk types (including whole, full cream, lite and skim) and brands (including Dairy Farmers, Devondale, Farmdale, Farmland, Perfection, Pura and Woolworths).

Iodine concentrations were highly variable. Median concentrations were 140 μg/L in 2001 (range, 60–220 μg/L) and 195 μg/L in 2004 (range, 66–412 μg/L). Iodine concentrations varied between samples of the same brand and type by up to 100 μg/L. Many samples contained less than 200 μg/L (10/13 in 2001 and 7/13 in 2004).

A 1975 survey of iodine concentration in milk conducted by the Australian Consumers’ Association found mean concentrations of 593.5 μg/L and 583 μg/L in NSW and Victoria, respectively.5 Because of concerns about iodine toxicity, Food Standards Australia and New Zealand specified an iodine limit of 500 μg/L in the Food Standards Code 1982. The replacement of iodophors by other sanitisers in the dairy industry appears to be the reason for the decrease in iodine content of Sydney milk. The perception that milk is a rich source of iodine is no longer true. A cup (250 mL) of milk a day would provide at most 50–60 μg iodine, approximating a third of the daily requirement for an adult.

We suggest that the reduced amount of iodine in milk is likely to be one of the explanations for the re-emergence of iodine deficiency in Sydney and perhaps elsewhere in Australia. Despite these changes, dairy milk remains an important source of dietary iodine. The iodine content in milk should be monitored.