FDG PET/CT findings of thyrotoxicosis induced thymic hyperplasia associated to interferon.

Interferon-alpha (IFN ) is a therapeutic agent which is used for infectious and malignant diseases. Side effects such as flu-like symptoms, hematologic abnormalities and thyroid disease have been reported associated with this therapy. Thyroid disease manifests as development of thyrotoxicosis and it may be secondary to destructive thyroiditis or Graves’ disease. Here we report a 61-year-old female patient with malignant melanoma. After an IFN treatment duration of 18 months, she started to complain of nervousness, sweating and palpitation. Serum free T3, free T4, thyroid stimulating hormone (TSH) and antithyroglobulin antibody (TGAb) levels were 22.27 pg/ml (normal: 2.5–3.9 pg/ml), 5.14 ng/ml (normal: 0.61–1.12 ng/ml), 0.05 IU/mL (normal: 0.34–5.6 IU/mL) and 312.5 IU/mL (normal: 0–115 IU/mL), respectively. Thyroid scintigraphy with 99mTc pertechnetate was consistent with diffusely increased uptake throughout the thyroid. FDG PET/CT imaging, which was performed for restaging of the melanoma, revealed anterior mediastinal mass in thymus location with elevated FDG uptake (SUVmax 2.91). Thyrotoxicosis induced thymic hyperplasia must be kept in mind in the differential diagnosis of an anterior mediastinal mass which obviates additional diagnostic work-up. To our knowledge, this is the first report in which FDG PET/CT findings of IFN associated thyrotoxicosis induced thymic hyperplasia have been shown (Figs. 1 and 2). Although disappearance of metabolic activity in subsequent FDG PET/CT evaluations could not be demonstrated, thymic rebound, which was attributed to thyrotoxicosis induced thymic hyperplasia, was thought. Thymic enlargement is a rare phenomenon in adults and is mainly observed after chemotherapy.1 Thyrotoxicosis may also cause thymic enlargement which is partly due to the stimulatory effect of raised circulating thyroid hormones2 and partly due to the binding of immunoglobulins to the thymocytes.3 Formation of thymic medullary lymphoid follicles and thymic hyperplasia is