PURPOSE: To develop a strategy to identify cases of endogenous subclinical 
hyperthyroidism and free triiodothyronine (free T3) 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 T4) 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 T3 toxicosis if they had: 
(1) a markedly subnormal TSH level (less than or equal to 0.1 mU/L), (2) a 
normal free T4, (3) a normal total T3, (4) evidence of a primary thyroid 
abnormality (e.g., autonomous function on a thyroid scan), and (5) an elevated 
free T3 level by tracer equilibrium dialysis. Patients meeting conditions 1-4, 
but with normal free T3 levels, were considered to have subclinical 
hyperthyroidism.
RESULTS: One thousand twenty-five patients had TSH and simultaneous free T4 
determinations, and 148 of these had markedly subnormal TSH but normal free T4 
levels. Three patients met the criteria for free T3 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 T4 and total T3. If these are 
normal, a T3 level (by tracer equilibrium dialysis) be obtained to distinguish 
subclinical hyperthyroidism from overt free T3 toxicosis. A thyroid scan and 
radioiodine uptake measurement can be obtained to substantiate the diagnosis. 
Some patients with these conditions will benefit from treatment.