SE5SYNOPSIS A rapid and precise radioimmunoassay (RIA) for serum total thyroxine (T4) on as little as 1-10 pl of unextracted serum is described. Results in hypothyroidism (overt and borderline), in euthyroid subjects, in pregnant and oestrogen-medicated subjects, and in hyperthyroidism (overt and borderline) are compared with the results on the same sera by an established competitive proteinbinding technique (Ames' Tetralute) on unextracted serum from a different laboratory. The correlation between the two methods was excellent (r = 0 94) and no significant difference between overall results or results in any sulYgroup was found. In the hypothyroid range the radioimmunoassay appeared to measure total T4 reliably in sera containing only 1'3 or 2-6 nmol/l. Both methods predicted the clinical outcome in borderline hypothyroidism and borderline hyperthyroidism equally well and both gave normal results in T3-toxicosis. It is concluded that both techniques reliably measure total T4. RIA appears to have advantages of sensitivity and precision (especially in the hypothyroid range), of simplicity, and of low cost.The determination of serum total thyroxine (T4) is still the most widely used single measurement employed in the clinical assessment of thyroid function (Chopra, 1972 (Chopra, 1972;Mitsuma et al, 1972;Beckers et al, 1973). However, discrepancies have been observed between the values of total T4 found by RIA and by CPB techniques. Chopra (1972) found that values in euthyroid and hyperthyroid subjects were higher by RIA; the difference was not thought to be due simply to losses in the extraction procedure employed in the CPB technique and was attributed to the presence in some sera of T4 which was linked by covalent bonds to serum proteins and not extracted by butanol/ethanol.
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