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Commentary

Endocrine Section, Washington Hospital Center, and Department of Medicine, Georgetown University, Washington, DC.

Address correspondence to the author at: Section of Endocrine, Washington Hospital Center, 110 Irving Street, NW, Room 2A-72, Washington, DC 20010-2975. Fax 202-877-6588; e-mail kenneth.d.burman@medstar.net

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Discordant measurements of serum triiodothyronine (T₄),¹ thyroxine (T₃), and thyroid-stimulating hormone (TSH) should always raise suspicions for unusual conditions. The clinical condition of the patient is especially relevant and should help guide further laboratory investigation. In the vast majority of instances, when a patient has authentic clinical hyperthyroidism with an increased T₄ or T₃, serum TSH should be low or undetectable (i.e., <0.01 mIU/L). Van Der Watt et al. nicely describe the possible causes of an increased serum free T₄ (FT₄) with a normal FT₃ and TSH. I would like to emphasize several specific circumstances. A TSH-secreting pituitary tumor can mediate hyperthyroidism although serum TSH is low or normal (1). TSH bioactivity depends on proper glycosylation of the TSH molecule, and this may be altered in patients with a TSH-secreting pituitary tumor or a nonfunctional pituitary tumor (1). In . . . [Full Text of this Article]