Abstract
The " occupancy principle " states that in a rheostatic (constant-flow) system the ratio of occupancy to capacity is the same for all parts of the system. In a worked example, in which the total iodine content of the thyroid (i.e., the capacity of the thyroid) is the unknown, a single dose of iodine-129 is used as a tracer. The areas under the activity-time curves for the thyroid and for 1 litre of plasma give the occupancies; plasma capacity is simply the protein-bound-iodine content of 1 litre of plasma. Equating the occupancy/capacity ratios for plasma and thyroid gives a value of 5·6 mg. for total thyroid iodine content. Iodine content can be estimated differently, by using the peak plasma activity and the thyroid activity at this time — in this example, the result was 5·5 mg.
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