Objective First-line treatment of thyrotoxicosis in young people is thionamide anti-thyroid drug (ATD) in a blocking dose with levothyroxine replacement (block and replace, BR) or in a smaller dose tailored to render the patient euthyroid (dose titration, DT). Our objective was to determine which regimen provides more stable biochemical control. Design A multi-centre phase III, open-label randomised trial comparing BR with DT in patients aged 2–17 years with newly diagnosed thyrotoxicosis at 15 UK centres. Methods Patients were randomised shortly after diagnosis and treated for 3 years. The primary outcome was the percentage of serum thyroid-stimulating hormone (TSH) levels in the reference range between 6 months and 3 years. Secondary outcomes included the proportion of Free thyroxine (FT4) levels in the reference range, adverse event frequency and 4 years outcome (remission/relapse). Results Eighty-two patients were randomised, with details on clinical course in 81 (62 Female); 40 were allocated to BR (41 DT). Three withdrew with one ineligible. The mean percentage of serum TSH within reference range was 60.2% in BR and 63.8% in DT patients; adjusted difference 4.3%, 95% CI (−7.8 to 16.4); P = 0.48. Proportions for FT4 were 79.2% in BR and 85.7% in DT patients; adjusted difference 6.8%, (−0.2 to 15.6); P = 0.13. Three patients developed neutropenia – all on BR. 6 BR and 10 DT patients were in remission at 4y. Conclusion This randomised trial has shown no evidence to suggest that BR, when managing the young patient with thyrotoxicosis, is associated with improved biochemical stability when compared to DT.
