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Background Radioiodine is the treatment of choice for relapsed hyperthyroidism although the optimum protocol is uncertain. Fixed dose radioiodine is increasingly popular but responses may vary. Aim To assess the outcome of 131I therapy in hyperthyroidism using a standard dose regimen in a regional referral centre and to explore factors influencing outcome. Methods We studied 449 patients (M:F 82:367; age range 13-89y, median 42y) with hyperthyroidism treated between 2003 and 2007 with a standard dose of 550MBq 131I. Patients were classified as either Graves’ disease, toxic multinodular goitre or indeterminate aetiology. Antithyroid drugs were routinely stopped at least 1 week before radioiodine. Results One year after radioiodine 334 (74%) were hypothyroid, 85 (19%) were euthyroid and 30 (7%) had required a further dose of 131I. Patients with Graves’ disease were more likely to become hypothyroid than those with toxic multinodular goitre (78% v 37%, p80pmol/L (normal range 9.0 – 19.0 pmol/L) at presentation was associated with an increased failure rate (17% compared with 5% and 3% for 40-79pmol/L and

A double antibody heterologous enzyme immunoassay was developed for measuring thyroxine (T4) in serum and in dried blood samples on filter paper for use in screening for neonatal hypothyroidism. In this method, ethanol extracts of 100 microliter of serum or dried blood spots of 9 mm in diameter were incubated with T4-alkaline phosphatase conjugate prepared with glutaraldehyde, and antiserum to T4-bovine serum albumin conjugate prepared with carbodiimide. The enzyme activity in the precipitates formed with second antibody was then determined photometrically at 500 nm. The measurable range of T4 was 1-32 micrograms/dl. A linear relation was obtained between the sample volume and the T4 value, and a high correlation was found between values for T4 determined by this method and those for T4 in serum determined by radioimmunoassay (coefficient of correlation: r=0.97 for serum and r=0.88 for dried blood samples).