Physiological aspects of the thyroid trapping function and its suppression in iodine deficiency using 99mTc pertechnetate.

In an area of iodine deficiency, we investigated 190 individuals with and without euthyroid endemic goitre, who had a normal TSH response after TRH and an entirely homogeneous thyroid scintigram before and under suppression. In these subjects, the thyroid uptake of 99mTc pertechnetate, as a measure of the iodide trapping function, was determined before (TcUb) and under suppression (TcUs), using quantitatively evaluated scintigraphy. In this control group of individuals, without evidence of autonomy, the reference ranges of TcUb and TcUs were determined. The upper limit of the reference range for TcUb was 7.4% of the tracer activity injected, and for TcUs 1.6%. The reference range of TcUs is to be used to detect accurately thyroid autonomy in vivo. In addition, factors affecting the thyroid trapping function were investigated. With decreased iodine supply, trapping before suppression was increased, compensating for iodine deficiency. The effect of TSH on the trapping function was secondary, indicating that, to a considerable degree, the follicular cells adapt their iodide trapping to the iodine supply. The trapping before and under suppression increased with the estimated thyroid weight. delta TSH after TRH stimulation measured before suppression correlated inversely with the trapping under suppression. The two latter observations suggest that there is a continuum between low and increased levels of the TSH-independent thyroid function, even among these selected individuals without primary evidence of autonomous tissue. A correlation of the trapping function with sex, oestrogen treatment, and goitre type was not demonstrable. Age was found to have a small influence. Except for iodine contamination, the factors affecting the pertechnetate uptake of the thyroid, can be neglected under routine conditions.