Diffusely increased thyroidal uptake of F-18 fluorodeoxyglucose in a patient with non-Hodgkin's lymphoma and Graves' disease.

Increased uptake of F-18 fluorodeoxyglucose (FDG) has been reported in thyroiditis and hypothyroidism. The authors present a case where increased FDG uptake in the thyroid was subsequently corroborated with a pertechnetate scan and thyroid hormone levels to diagnose previously undetected Graves' disease in a patient of non-Hodgkin's lymphoma being followed-up with positron emission tomography for disease recurrence.

INTRODUCTION

Planar thyroid scintigraphy showing increased 99mTcO4 (pertechnetate) uptake is the hallmark of Graves’ disease, and is used to differentiate this condition from thyroiditis. However, diffuse uptake of F-18 fluorodeoxyglucose (FDG) in the thyroid is usually associated with thyroiditis. This case illustrates an alternate clinical possibility for increased thyroidal FDG uptake.

CASE REPORT

A 63-year-old female patient with non-Hodgkin's lymphoma in remission for the last 2 years after chemotherapy was referred to our department for a whole body FDG-positron emission tomography (PET)/computed tomography scan to assess for disease recurrence after recent onset of diplopia and raised β2-microglobulin levels. Multiple lymph nodes in the cervical, mediastinal and right axillary regions showed increased metabolic activity, suggestive of recurrence. In addition, diffusely increased FDG uptake was seen in both lobes of the thyroid gland on the whole body PET image [Figure 1a], with a standardized uptake value of 12.3. On clinical examination, she was found to have bilateral exophthalmos and the biochemical profile showed high T3 and T4 and low thyroid stimulating hormone values, suggestive of hyperthyroidism. Subsequently, a 99mTcO4 (pertechnetate) scan [Figure 1b], also showed diffusely increased tracer uptake in the thyroid gland; uptake at 20 min was 6.1%. A diagnosis of Graves’ disease was made and treatment with Carbimazole was started.

Figure 1

(a) Whole body fluorodeoxyglucose (FDG)-positron emission tomography (maximum intensity projection image) showing diffusely increased uptake of FDG in both lobes of the thyroid gland. Multiple FDG avid axillary and mediastinal nodes are also seen. (b) 99mTc pertechentate scan showing homogenously increased tracer uptake in both lobes of the thyroid gland

DISCUSSION

The value of FDG PET for the diagnosis of several diseases affecting the thyroid gland has been demonstrated, and there is a growing evidence that this investigation may help in differentiating malignant from benign disease.[12] With increasing use of whole body FDG-PET in clinical practice, incidental focal or diffuse FDG uptake in the thyroid is being detected more frequently. Diffusely increased FDG uptake in the thyroid has been reported in 0.6-3.3% of the cases. Although, some authors believe that this may be a normal occurrence,[3] other studies suggest that such uptake is primarily associated with autoimmune thyroiditis or hypothyroidism.[4] Diffuse thyroidal FDG uptake is most commonly benign and is usually caused by chronic lymphocytic (Hashimoto's) thyroiditis.[5] In a few other cases, diffuse thyroid FDG uptake has also been linked to Graves’ disease.[67] The added visualization of the thymus gland and FDG uptake in the skeletal muscles has been described as characteristic of Graves’ disease.[89] The present case shows that increased FDG uptake in the thyroid may also be seen in this condition without any thymic or skeletal muscle involvement. The exact mechanism of increased thyroidal uptake of FDG is not well-established. Graves’ disease with significant infiltration and activation of T lymphocytes in the thyroid and thymus can induce upregulation of Glucose transporter 1 (GLUT1), which is the major isoform of glucose transporter in the T lymphocytes.[10]