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Radiation Dosimetry of Radioiodinated Thyroid Hormones

VA Medical Center, Palo Alto, California
Departments of Radiology (Nuclear Medicine), Medicine, and Surgery UC Davis School of Medicine, Sacramento, California

Correspondence: For reprints contact: Marguerite T. Hays, MD (151), VA Medical Center, 3801 Miranda Ave., Palo Alto, CA 94304.

ABSTRACT

A physiologically based compartmental model for T₄ and T₃ metabolism in man was used to generate time-activity curves for residence of radioiodine in key organs. T₄ and T₃ labeled with¹²³I, ¹²⁴I, ¹²⁵I, and ¹³¹I were studied. Conditions modeled Included radioactive iodine uptake (RAIU) values of 0%, 1%, 5%, 15% and 25%, and RAIU of 15% combined with various degrees of pharmacologic block of thyroidal RAIU. Using the MIRD "S" tables, red doses were generated for each condition. While the shapes of the time-activity curves varied widely with alterations in physical and biological turnover and with changes in steady-state due to iodine administration, it was possible to calculate overall effective half-lives for each organ of interest from the integral of the time-activity curve projected by solution of the model. This overall effective half-life of the hormone for the body's exchangeable hormone compartments correlated well with calculated radiation dose to the thyroid in the unblocked state. With progressive degrees of iodine block, this correlation persisted, though with proportionately reduced thyroid radiation doses. Use and manipulation of a compartmental model, rather than the usual multiexponential model, for radiation dosimetry facilitates conceptualization and the projection of the effects of interventions such as iodide block.