A specific, accurate, precise and simple radioimmunoassay for thyroxine (T4) in small volumes of unextracted serum is described. It discriminates well between levels found in different states of thyroid function. Levels in euthyroid subjects range between 4.3 and 11.2 μg/100 ml (mean 7.7); in untreated hyperthyroid subjects, 13.2‐25.6 μg/100 ml (mean 19.2); and in untreated hypothyroid subjects undetectable to 2.6 μg/100 ml (mean 1.4). There is satisfactory correlation with competitive protein binding and protein bound iodine methods. Because of its advantages over previous techniques, it is suggested that T4 radioimmunoassay will become the routine method of assessing thyroid status.The measurement of serum thyroxine (T4) is the single most important test in the routine laboratory assessment of thyroid status. Ideally the T4 assay should be specific, cheap, simple and have a high sample capacity. Thyroxine is determined most commonly in serum extracts by competitive protein binding (CPB) methods (Murphy & Pattee, 1964; Ekins et al., 1969) or, indirectly, by estimation of serum protein bound iodine (PBI). Both techniques require the processing of relatively large volumes of serum before assay. The PBI method has the added disadvantage of measuring iodine‐containing compounds other than thyroxine (Acland, 1971).We report here a specific, precise and simple radioimmunoassay for thyroxine in unextracted human serum and its application to the assessment of thyroid status. It has significant advantages over CPB, PBI and previous radioimmunoassay methods.

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SUMMARY1. Feeding male rats 02 % dinitrophenol (DNP) in the diet caused weight loss, a significant fall in the plasma protein bound iodine concentration, and a rise in the thyroxine secretion rate. Thyroxine concentration was raised in the liver and intestinal tract with contents, reduced in the carcass, and unchanged in the pelt.2. Radio-iodine uptake by the thyroid gland was unchanged but there was a significant fall in the thyroid/serum activity ratio.3. Five hours after tracer radiothyroxine, DNP treated rats had a reduced fraction of dose in the plasma, with raised concentrations in the liver and intestines.4. Fractional turnover rates of radiothyroxine in plasma, liver and carcass were raised in DNP treated animals.5. DNP caused a significant increase in the biliary clearance of radiothyroxine, when measured immediately after administration of tracer and 4 hr later.6. It is postulated that DNP displaces thyroxine from its plasma binding proteins and that the displaced hormone is excreted in the bile. This wastage together with increased demand resulting from DNP induced hypermetabolism results in increased output of thyroxine by the thyroid gland.

Thyroxine secretion rates and thyroxine concentration in various organs (liver, spleen, intestines, kidneys, heart and thymus) have been estimated in five strains of mice by a technique employing multiple radiothyroxine tracer injections. Marked interstrain differences have been demonstrated. No correlation was found with physical activity or with catabolic rates for the soluble protein antigen, bovine gamma-globulin.