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The Estimation of Extra-Thyroid Neck Tissue Radioactivity During Thyroid Radioiodine Uptake Measurements¹

Sydney, New South Wales, Australia

ABSTRACT

Two methods of estimating the extra-thyroid tissue radioactivity, which is detected simultaneously by the neck probe during measurements of thyroid radioiodine content, have been assessed. In 42 subjects with no thyroid ¹³¹I uptake this extra-thyroid radioactivity was directly measured at 2.5, 30, 60, 90, and 120 minutes after the intravenous administration of the dose. The overall mean value was .025 of the total extra-thyroid radioactivity in the body, with a standard deviation of ± .0065. Slightly higher values were observed in patients with diffuse toxic goitre in whom thyroid uptake had been blocked by perchlorate, than in euthyroid persons.

In 61 subjects in whom there was unimpaired uptake of ¹³¹I by the thyroid, the proportion of total extra-thyroid radioactivity in the neck tissues was determined by calculation using the method described by Oddie. In normal subjects, the overall mean value was similar to that obtained by direct measurement and there was a significant degree of correlation between the calculated and measured values separately obtained in 14 individuals. However, in TSH stimulated normals, in patients with non-toxic goitre and in thyrotoxic patients, the mean calculated values at both 30 and 60 minutes were significantly higher than those for normals.

Accordingly, although calculation provides a rather more exact estimate of the extra-thyroid neck tissue radioactivity in any normal individual, it is considered that the mean value obtained by direct measurement in a group of subjects should be used when studying patients with suspected abnormal thyroid function.

FOOTNOTES

¹ From the School of Medicine, University of New South Wales and the Thyroid Unit, The Prince of Wales Hospital, Randwick, New South Wales, Australia.