Abstract
purpose: To develop a strategy to identify cases of endogenous subclinical hyperthyroidism and free triiodothyronine (free T 3) thyrotoxicosis in otherwise healthy ambulatory patients. patients and methods: In a retrospective study we reviewed the records of ambulatory patients who had thyroid stimulating hormone (TSH) levels determined between October 1, 1991 and August 31, 1992. Each patient also had a simultaneous free thyroxine (free T 4) measurement. Patients were excluded from consideration if they had active, concurrent non-thyroidal illness, psychiatric disease, known hypothalamic/pituitary lesions, were under treatment for hyper- or hypothyroidism, were on drugs known to affect TSH levels, or were pregnant. Patients without exclusions were diagnosed with free T 3 toxicosis if they had: (1) a markedly subnormal TSH level (less than or equal to 0.1 mU/L), (2) a normal free T 4, (3) a normal total T 3, (4) evidence of a primary thyroid abnormality (e.g., autonomous function on a thyroid scan), and (5) an elevated free T 3 level by tracer equilibrium dialysis. Patients meeting conditions 1–4, but with normal free T 3 levels, were considered to have subclinical hyperthyroidism. results: One thousand twenty-five patients had TSH and simultaneous free T 4 determinations, and 148 of these had markedly subnormal TSH but normal free T 4 levels. Three patients met the criteria for free T 3 toxicosis and three had subclinical hyperthyroidism. All six patients had either multinodular glands or a single nodule on thyroid exam. Four patients were treated with radioactive iodine or surgery, resulting in reversal of the TSH suppression in three cases. conclusion: Apparently healthy ambulatory patients with subnormal TSH levels should be worked up with measurements of free T 4 and total T 3. If these are normal, a T 3 level (by tracer equilibrium dialysis) be obtained to distinguish subclinical hyperthyroidism from overt free T 3 toxicosis. A thyroid scan and radioiodine uptake measurement can be obtained to substantiate the diagnosis. Some patients with these conditions will benefit from treatment.
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