SUMMARYSerum thyroxine (T4), thyroxine‐binding globulin (TBG) and T3 Uptake have been measured in 122 hospital inpatients, 116 healthy volunteers, 101 patients with thyrotoxicosis, thirty‐six patients with myxoedema, eighty‐five euthyroid subjects with high or low TBG concentration and ten patients on treatment with diphenylhdantoin (DPH). T4:TBG ratio and free thyroxine index (FTI = T4/T3Ux 100) were calculated for each individual. In euthyroid controls, TBG concentration was normally distributed, and was significantly greater in females (12.1 ± 1.6 mg/l, mean ± SD) than in males (10. 8 ± 2.9 mg/l). TBG was raised in myxoedema (14.7 ± 2.4 mg/l), and reduced in thyrotoxicosis (10.8 ± 2.4 mg/l) when compared with euthyroid females. There was a 24%, rise in TBG concentration during drug treatment of thyrotoxicosis (pre‐treat‐ment= 11.1 ± 2.5 mg/l, post treatment (3/12) = 13.8 ± 3.4 mg/l). In euthyroid persons, T4 was linearly correlated with TBG concentration (r = 0. 92), but there was a tendency towards a curvilinear relationship which may have been due to reduction in the concentration of thyroxine‐binding prealbumin (TBPA) at high concentrations of TBG. T4:TBG ratio and FTI were similar in their ability to distinguish between thyrotoxic, euthyroid and myxoedematous persons when TBG concentration was normal. T4:TBG ratio was found to have the considerable advantage of correcting for increases in TBG concentration over a five‐fold range, whereas FTI was misleading when TBG concentration was raised. Both FTI and T4:TBG gave unreliable results when TBG was low, and in such cases measurement of TBG should alert the clinician to the need for alternative tests. During DPH treatment, both indices tended to give‘myxoedematous’results. Retrospective analysis of T4:TBG ratio and FTI in 104 hospital in‐patients with T4< 110nmol/l and 40 in‐patients with T4<40nmol/l suggested that the T4:TBG ratio gave better diagnostic precision than FTI.

The concentration of thyroxine-binding globulin (TBG) in the serum can now be measured by direct assays that are simple and inexpensive. Comparison of a direct measurement of TBG concentration with a widely used indirect method (Thyopac-3) showed that the indirect method was inaccurate when TBG concentrations were high. This will result in an increase in the derived free thyroxine index (FTI), so that euthyroid patients with a raised TBG concentration may be at risk of being labelled thyrotoxic. Correction of serum total thyroxine (T4) concentration according to the actual TBG concentration (T4:TBG ratio) provided a better correlation with thyroid state than FTI.