Infiltration of the thyroid gland by non-thyroid malignancy: A literature review reveals this to be an unusual cause of hyperthyroidism

Background: Non-thyroid malignancies that metastasize to the thyroid gland are relatively rare. At one end of the spectrum they may only be identified at the time of autopsy. At the other extreme, they may be identified during the evaluation of a progressive malignancy. Most patients who are identified as having metastases to their thyroid gland are euthyroid, but some patients may have associated hypothyroidism or hyperthyroidism. This review examines cases of hyperthyroidism associated with metastases affecting the thyroid gland. Results: Twenty four articles describing 26 cases of malignancy-associated hyperthyroidism were identified, with the cases presenting with features suggestive of a thyroiditis and with goiter. The solid malignancies (19 cases) were mostly breast and lung cancer. Hematologic malignancies (7 cases) were also reported with a similar thyroiditis-like presentation. Patients underwent the traditional work-up for a thyroiditis, but frequently underwent other radiographic studies, in addition to radioactive iodine imaging, and frequently also underwent thyroid biopsy. The course in most patients (22/26 cases) was progression from hyperthyroidism to hypothyroidism, as the underlying malignancy progressed or thyroidectomy was performed, or the patient succumbed to their malignancy. Some patients (4 cases) became euthyroid with successful treatment of their malignancy. A subset of patients (5 cases) initially presented with severe thyrotoxicosis. Many affected patients succumbed to their underlying malignancy. Conclusion: Malignancy-associated hyperthyroidism has a similar underlying mechanism to subacute thyroiditis, in so much as there is damage or destruction of thyroid tissue. In cases of subacute thyroiditis this damage is self-limited, and there is recovery of thyroid function. In some cases of thyroiditis associated with malignancy there may be thyroid gland recovery as the underlying malignancy is treated and controlled. However, if the malignancy progresses, eventual hypothyroidism is likely to ensue.