Irradiation Produced Rise in Blood Radioiodine Concentration Following Ingested Therapeutic Dose for Metastatic Thyroid Carcinoma.

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Abstract
Four to 8 days after ingestion of therapeutic doses of radioiodine (50-200 mc) by patients with metastatic thyroid carcinoma, a rise occurs in blood radioiodine concn. which is attributed to irradiation damage to the thyroid gland or thyroid tumor tissue. This increase is termed the irradiation produced rise. In a series of patients the magnitude of the IPR ordinarily increases during radiation thyroidectomy with the quantity of radioiodine retained by the thyroid. In a given patient both magnitude and incidence of the IPR decrease with successive doses as functioning thyroid tissue is progressively destroyed. Finally, when there is no longer any functioning tissue, as demonstrated by absence of I131 uptake, blood radioiodine rapidly falls to very low levels and there is no IPR. The blood of 21 patients, in various stages of therapy, was studied. For all of 7 radiation thyroidectomy doses, significant IPR''s were found; in thyroidectomized patients with functioning metastases, IPR''s were demonstrated in 8 of 33 doses. However, no such rise in blood I concn. was observed following 4 doses to thyroidectomized patients lacking demonstrable I131 uptake. Since no such rise is found in the absence of functioning thyroid tissue, either neoplastic or non-neoplastic, the occurrence of an IPR in a thyroidectomized patient may, therefore, be considered evidence of the existence of viable functioning thyroid carcinoma metastases.