PMC8250423_01
FDG-PET (A) shows a marked focal uptake with an SUV=14, corresponding to left thyroid lobe at PET/CT (B).
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Austrian male presented at the institute of nuclear medicine in Steyr, Austria, for evaluation of possible thyroid malignancy. A FDG-PET/CT examination from March 2019 showed a marked focal uptake in the left thyroid lobe only (see  Figure 1 ).
(A-F) Pembrolizumab induced thyroiditis: (A) H&E 200x: thyroid tissue with follicle partially destucted by histiocytes. (B) CD3 100x: thyroid tissue densely infiltrated by CD3 reactive T-lymphocytes. (C) CD 4 200x: thyroid tissue densely infiltrated by T helper cells (CD4 reactive). (D) CD 20 100x: thyroid tissue sparsely infiltrated by CD20 reactive B-lymphocytes. (E) CD 68 200x: thyroid tissue densely infiltrated and partially destructed by CD68 reactive histiocytes. (F) PD-L1 (SP263 hemalum2 8min) 400x: thyroid follicle partially destructed by histiocytes with coexpression of PD-L1 (clone SP263). (G) Regular thyroid tissue: PD-L1 (SP263 hemalum2 8min) 400x: normal thyroid tissue without PD-L1 (clone SP263) reactive cellular elements. (H, I) Graves  Disease: (H) CD68 reactive histiocytes in the stroma and in thyroid follicles. (I) Histiocytes in the stroma and in thyroid follicles showing coexpression of PD-L1 (clone SP263).
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Because of the medical history pembrolizumab induced thyroiditis was finally considered and evaluation of the immunological processes of the specimen was added (see  Figure 2 ).