Unexpected Tumor Regression After Recombinant Human Thyroid-Stimulating Hormone Administration

Differentiated thyroid cancer represents the majority of thyroid cancer cases, and recombinant human thyroid-stimulating hormone (rhTSH) is commonly used to facilitate diagnostic imaging and radioiodine therapy, including poorly differentiated disease. Although generally safe, several adverse effects have been reported, including residual tumor enlargement. The objective of this report is to describe a patient with an unexpected tumor regression following rhTSH administration.

A 77-year-old man with metastatic follicular thyroid cancer and multiple prior treatments received rhTSH in preparation for a whole-body scan to guide treatment. He had a known right thyroid bed lesion seen on prior CT imaging, with previously negative whole-body iodine scans. Shortly after rhTSH administration, he developed voice changes and mild dyspnea, prompting glucocorticoid treatment. Repeat CT imaging showed interval enlargement of the thyroid bed lesion. His symptoms resolved over the following days. On a follow-up CT scan obtained months later, the lesion was unexpectedly smaller.

This case adds to reports of rapid, transient tumor enlargement after rhTSH administration, likely driven by acute vascular and inflammatory changes and endothelial dysfunction. The clinical course, including improvement with glucocorticoids, suggests that lesion size changes may reflect inflammatory and vascular effects rather than true tumor growth.

This case illustrates that rhTSH may cause sudden tumor expansion, highlighting the potential role of vascular and inflammatory changes. When lesions are located in anatomically sensitive areas, close observation is warranted before rhTSH administration.