125I-fibrinogen: thyroid blocking with 2.5 mg potassium iodide twice daily in patients after hip surgery.

Blocking the thyroid gland when administering radioiodinated materials is a common practice. Since pharmacologic quantities of potassium iodide (KI) in the range of 100-300 mg daily are usually administered for this purpose, the potential exists for iodide toxicity. Concomitant with the tracer administration is the risk produced from ionizing radiation. Reports in the literature demonstrated thyroid function abnormalities in patients administered iodides. Based on the latter observations the object of the present investigation is to determine the effectiveness of thyroid blocking with a lesser daily quantity of KI. Fifteen adult hip replacement patients (7 men, 8 women; 58.2 +/- 11.5 years) received 2.5 mg Ki orally every 12th hour twice daily 2 days prior to surgery and 125I-fibrinogen [(3,700 kBq)(100 microCi)], and each day up to 10 days thereafter. Thyroid and precordial counts were obtained daily and the latter two were used for calculating the 125I thyroid uptake. These data were used for estimating the thyroid blood disappearance and cancer risk. An attempt was made to compare the radiation risk to that from KI ingestion. The 125I% thyroid uptake for the study population (n = 15) was 1.83 +/- 1.25%. This compares to a thyroid uptake of 0.064 +/- 0.037% in a published report using 300 mg KI daily in the course of an 125I-fibrinogen test. The mean thyroid radiation dose for the study population was 6.09 cGy after receiving 5 mg KI daily. The specific risk estimate (SRE) for contracting thyroid cancer was calculated to be 4.5B-04. The NCRP calculated thyroid function risk after ingesting KI is 1-10E-07.(ABSTRACT TRUNCATED AT 250 WORDS)