Effect of Recent Contrast-Enhanced CT and Patient Age on Image Quality of Thyroid Scintigraphy

Purpose of the Report: When thyroid scintigraphy (TS) is performed after contrast-enhanced CT (CE-CT), tracer uptake of (TcO4)-Tc-99m in the thyroid gland can be inhibited by free iodide. Currently, it is recommended to postpone TS until 4 to 8 weeks after CE-CT, but few data exist to support this recommendation. The purpose was to investigate the effect of CE-CT and other variables for the diagnostic quality of TS. Patients and Methods: This retrospective study included 196 patients subjected to TS less than 3 months after a CE-CT (median, 66 days). Patients with elevated thyroid-stimulating hormone (>4.5 mIU/L) or suspected thyroiditis were excluded. Logistic regression was used to calculate the probability of a TS of diagnostic quality with the variables days since CE-CT, age, thyroid-stimulating hormone, and kidney function (eGFR). Results: Days since CT and age were highly significant (P < 0.001) predictors for diagnostic TS. The probability of diagnostic quality TS after CE-CT increased with time and reached approximately 70% to 80% 6 to 8 weeks after CE-CT. Analysis of age-specific populations showed age to be a strong independent factor. Conclusions: Our findings are in consensus with the currently recommended interval of 6 to 8 weeks between CE-CT and TS. However, our results indicate that patient age should be taken into account, and we suggest the following delay from CE-CT to TS: 4 weeks for patients aged younger than 50 years, 6 weeks for patients aged 50 to 60 years, and 8 weeks for patients aged older than 60 years.