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Plasma thyroxine concentrations were measured by radioimmunoassay in plasma samples from 691 lactating ewes in 26 areas of New South Wales, Queensland, Western Australia and Tasmania. Sheep sampled in New South Wales and Tasmania had significantly lower plasma thyroxine values (4.0 and 4.3 microgram/100 ml respectively) than those sampled in Queensland and Western Australia (5.4 and 5.3 microgram/100 ml respectively). However, sheep in some districts in southern Queensland also had low plasma thyroxine values. The areas where sheep had low plasma thyroxine values correlate well with areas where goitre has been previously reported, both in man and in domestic animals. This suggests that measurement of plasma thyroxine is probably a valid empirical method of assessing the relative iodine deficiency of grazing sheep and further that sheep grazing substantial areas of New South Wales, Tasmania and to a lesser extent Queensland may have thyroid dysfunction of varying degrees of severity. These findings could have implications for animal production in these areas.