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Simultaneous Measurement of Thyroidal Trapping (^99mTcO₄^–) and Binding (¹³¹I^–): Clinical and Experimental Studies in Man^*

Marguerite T. Hays and Betty Wesselossky

University of California at Los Angeles
State University of New York at Buffalo
VA Hospitals, Los Angeles, California and Buffalo, New York

Correspondence: For reprints contact: M. T. Hays, Nuclear Medicine Service, Veterans Administration Hospital, 3495 Bailey Ave., Buffalo, N.Y. 14215.

ABSTRACT

Thyroidal uptake was measured repeatedly after simultaneous intravenous administration of ^99mTcO₄^– and ¹³¹I^– to assess the thyroidal trapping mechanism (^99mTcO₄^–) as well as combined trapping and binding (¹³¹I^–). Standard thyroid uptake equipment was used. Studies in controls (n = 43) showed a mean thyroidal (neck minus thigh) ^99mTcO₄^– uptake of 3.5% reached within 10–20 min, followed by a gradual decrease. Iodine-131^– uptake curves were normal. In hyperthyroidism (n = 9), uptake of both radionuclides was increased. In untreated hypothyroidism and in goiter with low ¹³¹I^– uptake (n = 7), ^99mTcO₄^– uptake appears by this technique to be greater than the ¹³¹I^– uptake.

In a series of normal volunteers, thyroid space (from T/S ratio) as well as thyroidal uptake were determined. TSH administration (n = 7) stimulated the thyroidal uptake and the thyroid space of both radionuclides. Triiodothyronine (n = 7), while markedly reducing ¹³¹I^– thyroidal uptake and space, had no effect on measurements of thyroidal ^99mTcO₄^– concentration by this technique although suppression could be demonstrated by a more specific technique. Propylthiouracil (n = 5) also reduced ¹³¹I^– thyroidal uptake and the ¹³¹I space. It did not affect ^99mTcO₄^– uptake but increased the ^99mTc space. Lugol's solution (n = 6) virtually obliterated both thyroidal uptake and space for both radionuclides.

Validating studies comparing this technique with a scanning technique which identifies specific thyroidal ^99mTcO₄^– showed that the thigh correction is inadequate and that failure to demonstrate suppression after triiodothyronine was due to technical limitations. In seven control ^99mTcO₄^– studies over a 3-month interval on a single subject, the coefficient of variation for the technique used in these studies was 18.3%. For the scanning technique, it was 28.3%.

This method uses generally available equipment and is useful in detecting the stimulation of trapping seen in hyperthyroidism and after TSH stimulation. it should also be valuable in the study of iodine-deficient populations. However, caution is advised when uptake levels are low because of the large, variable concentration of pertechnetate in nonthyroidal neck tissues. To study low ^99mTcO₄^– uptakes, techniques which specifically identify thyroidal radioactivity must be used.

FOOTNOTES

^* Presented in part at the American Thyroid Association, 1967 and the Third International Congress of Endocrinology, 1968 (1,2).

Present address : Dr. Wesselossky, Urapol a Amadores #39-2, La Pastora, Caracas, Venezuela.

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