A patient with widely metastatic differentiated thyroid cancer who had been heavily pretreated with (131)I was given recombinant human thyroid stimulating hormone (rhTSH) prior to (131)I treatment. Clinical and physical data from both this case and the literature suggest that the recombinant hormone, not the (131)I, may have caused a significant portion of the tumor swelling, which in turn was the most likely cause of the patient's symptoms. The potential effect of (131)I-induced tumor swelling and direct radiation effect on the lung is also analyzed. We review the potential hazards associated with rhTSH in patients with metastasis and propose means of minimizing this risk.