Fever of Unknown Origin, Iodinated Contrast Media and Thyroid Scintigraphy

Sir,

We read with great interest the recent article by Mohanty et al. titled, “Thyroid scintigraphy in fever of unknown origin”.[1] The authors studied a patient with thyroiditis, who presented with fever and concluded that technetium (Tc-99 m) pertechnetate thyroid scan was useful for confirming the diagnosis. Indeed, atypical presentations of thyroiditis, especially those causing fever of unknown origin (FUO), may pose significant diagnostic challenges. Computed tomography (CT) with iodinated contrast media is one of the first imaging examinations in FUO; the patient presented had undergone a contrast-enhanced CT of the thorax and abdomen as mentioned by the authors.[1] Besides thyroiditis, regardless of type, decreased uptake of sodium–iodide symporter (NIS)-targeting tracers (radioiodine and Tc-99 m pertechnetate) by the thyroid gland is noted due to previous iodine intake. In particular, radiographic iodinated contrast media compromise the uptake of NIS-targeting tracers for 1–2 months following intravenous administration,[2] hamper thyroid scintigraphy of patients with thyrotoxicosis, and consequently delay diagnosis. Diffuse thyroidal gallium-67 (Ga-67) and fluorine-18 fluorodeoxyglucose (F-18 FDG) uptake associated with subacute thyroiditis has been reported to be useful in detecting the focus of FUO.[34] Furthermore, thyroid accumulation of such oncotropic tracers, used commonly for the investigation of FUO, is not affected by iodine overload. An early and accurate diagnosis is important for the proper treatment of patients presenting with thyrotoxicosis. Therefore, in the context of FUO and previous administration of iodinated contrast media, alternative functional imaging with Ga-67 or F-18 FDG should be considered.