Thyroxine and Thyroxine-Binding Globulin: Discriminant Formulae for the Separation of the Diagnostic Classes of Thyroid Function

The biochemical classification of thyroid functional status is often accomplished by the use of the thyroxine to thyroxine-binding globulin (T4:TBG) ratio. It has recently been shown, however, that the reference range of the T4:TBG ratio varies with the concentration of TBG. This makes the T4:TBG ratio an unwieldy diagnostic index. A more accurate and economical way to define thyroid function using T4 and TBG measurements is by linear discriminant formulae. This is so because T4 varies linearly with TBG at the T4 reference limits over a wide range of TBG concentrations. Using the T4 reference ranges established by Attwood and Atkin (Ann Clin Biochem 1982; 19: 101–3), the following formulae are obtained:

T4 (nmol/l)—4·05 TBG (mg/l)>94·5 indicates hyperthyroidism;

T4 (nmol/l)—2·72 TBG (mg/l)<23·8 identifies hypothyroidism.