Background uptake activity is used as a reference to assess treatment response by positron emission tomography-computed tomography (PET/CT) with 2-deoxy-2-[F-18]fluoro- D-glucose (F-FDG). Prior studies have reported decreased liver and increased muscle F-FDG uptake in patients with hyperthyroidism. We hypothesized that hyperthyroidism and hypothyroidism might have inverse effects on F-FDG uptake on PET/CT.
We recruited 36 patients with hypothyroidism and 36 age and gender-matched euthyroid participants. We recorded patient factors and background mean standardized uptake values normalized by lean body mass from the aortic blood pool, liver, and muscle. We compared the patient factors and background standardized uptake values normalized by lean body mass between hypothyroidism patients and the controls. We performed a multivariate analysis to determine the best predictors of the 3 different background standardized uptake values normalized by lean body mass.
Patients with hypothyroidism had higher liver standardized uptake values normalized by lean body mass (1.77±0.33 1.58±0.26, P=0.009) and aortic blood-pool standardized uptake values normalized by lean body mass (1.21±0.22 1.11±0.20, P=0.040) than the controls. In contrast, the muscle standardized uptake value normalized by lean body mass (0.50±0.09 0.54±0.09, P=0.044) of the patients with hypothyroidism was lower than that of the controls. The serum level of thyroid-stimulating hormone was an independent predictor of liver standardized uptake values normalized by lean body mass (β=0.356, P<0.001) and blood-pool standardized uptake values normalized by lean body mass (β=0.288, P=0.001). The serum level of free triiodothyronine was an independent predictor of muscle standardized uptake values normalized by lean body mass (β=0.310, P=0.002).
PET/CT scans showed that hypothyroidism patients had increased liver and blood-pool F-FDG uptake and decreased skeletal muscle F-FDG uptake compared with euthyroid individuals. These alterations should be noted when a metabolic response to cancer treatment on PET/CT is determined.

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