An unusual finding of elevated free thyroxine (FT4) concentration coexistent with familial low thyroxine-binding globulin (TBG) is described in a euthyroid patient. Clinical and other laboratory tests were normal and consistent with a clinical diagnosis of euthyroidism. Low TBG (TBG capacity 3–10 μg/100 ml) was confirmed by various electrophoretic procedures and by TBG immunoassay (0.9 mg/100 ml). Serum T4 concentration was low (2.0–5.5 μg/100 ml). Triiodothyronine (T3) resin uptake was increased (88–93%) and the free thyroxine index was normal. Twenty-four-hour thyroidal 131I uptake was normal and varied between 13–20%. Serum thyroid-stimulating hormone (μunits) and T3 concentration (80 ng/100 ml) were also normal. Long-acting thyroid stimulator (LATS) was not present. Free T4 concentration measured by dialysis procedure was markedly elevated (9.2–12.1 ng/100 ml). Pulse T3 and T4 tracer kinetic studies by the single compartmental method revealed accelerated blood disappearance of T4 t1/2 = 3.7 days) but normal disappearance of T3 t1/2 = 2.1 days). Extrathyroidal hormonal iodine pool (569 (μg) was in the range of normals. T4 degradation rate (144 μg/70 kg/day) was moderately elevated over the euthyroid mean values, but was still close to or within the upper normal range. T3 degradation was normal (34 μg/day). Failure to develop hyperthyroidism in the presence of elevated free T4 levels, but with normal T3 concentration and dynamics, may suggest that T3, not T4, plays a major role in regulating metabolic activity; alternatively, euthyroidism in this low-TBG patient despite elevated free T4 concentration may simply be explained on the basis of a daily T4 disposal rate which was not clearly abnormal.