Thyroxine-binding globulin (TBG) excess with increased total thyroxine (T4) and triiodothyronine (T3) levels has not been thought to produce symptoms. We report on a white boy, initially seen at 4.3 years of age and observed for 4 years, who has short stature caused by the excess thyroxine binding. At his initial examination his thyroxine-binding globulin (TBG) levels were elevated (17 mg/dL), and he had a T4 level of 25.8 micrograms/dL, short stature, a bone age of 19 months, normal vital signs, and hyperthyroid-stimulating hormone (TSH) response to thyrotropin-releasing hormone (TRH) testing (maximal value 58 microIU/mL). Results of tests obtained during the next 6 months showed other abnormalities related to thyroid function. Tests showed the following values: T3 412 ng/dL, thyroid uptake 24%, and low T3 resin uptake. They also showed these values: an elevated basal TSH of 8.7 microIU/mL, a slightly low preejection period to left ventricular ejection time ratio of 0.29 (normal 0.35 +/- 0.04), and WISC-R IQ within normal limits. Because of the persistent short stature, T3 supplementation was started at age 7 years and gradually increased to 35 micrograms/d. The patient showed no thyrotoxic symptoms. Serum T4 level decreased from 25.8 to 4.2 micrograms/dL, T3 increased to 1,240 ng/dL, the TRH/TSH test result was suppressed (maximal level 1.8), and the preejection period to left ventricular ejection time ratio decreased to 0.24. Growth velocity increased by 65%. Both of the child's parents had normal thyroid test results. A younger brother also showed similar elevations of TBG level and even greater T4 values (36 micrograms/dL).(ABSTRACT TRUNCATED AT 250 WORDS)