69 year-old female patient with multiple myeloma had undergone sequential FDG PET-CT studies to assess for disease activity after chemotherapy.
1) What incidental finding of clinical importance is noted in three sequential scans?
2) What is the clinical significance of the variation of the intensity of FDG uptake FDG in the organ of interest with this ancillary finding?
No significantly metabolically active myelomatous disease is noted on the three sequential scans. However, incidental note is made of diffuse increased uptake in both lobes of thyroid gland. In this patient, this uptake was attributed to a known history of hypothyroidism.
Diffuse thyroid uptake attributed to hypothyroidism
In the initial scan, minimally increase FDG uptake is noted in both lobes of thyroid gland with a SUVmax of 2.2. The TSH measured the day earlier was 3.04 micro IU/ml (normal range 0.34-5.60 micro IU/ml). Subsequently, a second FDG study showed intensely increased uptake in both lobes of thyroid gland. TSH measured on the same day showed a value of 56 micro IU/ml with low free T4 levels of 0.48 (normal range 0.58-1.64) nanogram/dl consistent with inadequate supplementation.
Subsequently, her dose of levothyroixine was increased from 50 microgram to 75 micrograms, and her follow up PET/CT study done approximately 2 months after that revealed decreased uptake of FDG in the thyroid gland with an SUVmax of 2.4. The TSH measured at that time was 5.54 microIU/ml. So, in this case, the variation of FDG uptake was mostly related to increase in TSH-mediated metabolism of thyroid gland. This phenomenon has been described in patients with thyroid cancer where thyroid hormone withdrawal to increase TSH has been shown to increase the sensitivity of FDG-PET scan sensitivity in detecting I-131 negative recurrent/metastatic thyroid cancer.
The other possible explanation could have been an increase in inflammatory activity of thyroiditis. In this situation, TSH levels would be typically low to borderline low. The clinical relevance and take home message with this case is that it is always prudent to check TSH levels when an interval increase in diffuse uptake of FDG in thyroid gland is noted.
1) Inflammatory uptake from chemotherapy
2) Inflammatory uptake from thyroiditis
3) TSH-mediated metabolism of the thyroid gland.
Check TSH levels when you see increased, diffuse thyroid gland uptake.
1. Prestwich RJ, Viner S, Gerrard G, Patel CN, Scarsbrook AF. Increasing the yield of recombinant thyroid-stimulating hormone-stimulated 2-(18-fluoride)-flu-2-deoxy-D-glucose positron emission tomography-CT in patients with differentiated thyroid carcinoma. Br J Radiol. 2012 Oct;85(1018):e805-13.
2. Rothman IN, Middleton L, Stack BC Jr, Bartel T, Riggs AT, Bodenner DL. Incidence of diffuse FDG uptake in the thyroid of patients with hypothyroidism. Eur Arch Otorhinolaryngol. 2011 Oct;268(10):1501-4.