A 75-year-old woman with end-stage renal disease who is on chronic hemodialysis has a suppressed thyroid-stimulating hormone (TSH) of 0.1 µU/mL and levels of free thyroxine (T4) and free triiodothyronine (T3) in the mid-normal range confirmed on repeat testing. Her history includes a remote total thyroidectomy for benign disease. She is taking no thyroid medication. A thyroid scan indicates no uptake. Pituitary testing, whole-body I 131 scan, and antibody titers have not been checked. What is the next step in the workup of this patient?
—Harvey B. Ulano, MD, PhD, Stuart, Fla.

Following total thyroidectomy, hypothyroidism is expected to develop within two to four weeks (because of the seven day half-life of T4), and supplemental thyroid hormone is almost always necessary to achieve a euthyroid state. In the absence of medication, normal free T4 and free T3 levels, along with a suppressed TSH, suggest either the presence of overactive residual thyroid tissue or excessive ectopic production of T4. Since a thyroid uptake scan demonstrated no uptake in this patient, inadvertent subtotal thyroidectomy is unlikely, but ectopic thyroid tissue is still possible. You may want to consider thyroid scintigraphy (with I 123) to evaluate this further. Whole-body scanning with I 131 is typically reserved for postoperative evaluation of patients with a history of thyroid cancer and not with benign disease as in your patient, but such scanning may still be a consideration. You may also want to measure thyroglobulin levels, which should be undetectable with surreptitious use of thyroid hormone in the absence of residual tissue. Pituitary disease seems very unlikely since the combination of central hypothyroidism and total thyroidectomy should result in very low levels of free T4, not values in the mid-normal range. Given the complexity of this case, you may also want to enlist the assistance of an endocrinologist.
—Daniel G. Tobin, MD (116-15)

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