This paper summarizes the biokinetic database for iodine in the human body and proposes a biokinetic model for systemic iodine for use in dose assessments for internally deposited radioiodine. The model consolidates and extends existing physiological systems models describing three subsystems of the iodine cycle in the body: circulating inorganic iodide, thyroidal iodine (trapping and organic binding of iodide and synthesis, storage and secretion of thyroid hormones), and extrathyroidal organic iodine. Thyroidal uptake of inorganic iodide is described as a function of stable iodine intake (Y, µg day(-1)) and thyroidal secretion of hormonal iodine (S, µg day(-1)). Baseline parameter values are developed for reference adults with typical iodine intake. Compared with the current systemic biokinetic model of the International Commission on Radiological Protection (ICRP) for occupational intake of radioiodine, the proposed model predicts higher absorbed doses to the thyroid per unit uptake to blood for very short-lived iodine isotopes, similar absorbed doses to thyroid for iodine isotopes with half-life of at least a few hours, and substantially higher estimates of absorbed dose to stomach wall, salivary gland and kidneys for most iodine isotopes. Absorbed dose estimates for intravenous administration of radioiodine-labeled thyroid hormones based on the proposed model differ substantially in some cases from current ICRP values.