[1] N Staffordshire Hosp, Dept Clin Biochem, Stoke On Trent ST4 7PA, Staffs, England
[2] N Staffordshire Hosp, Dept Epidemiol, Stoke On Trent ST4 7PA, Staffs, England
[3] Univ Newcastle Upon Tyne, Dept Stat, Newcastle Upon Tyne NE1 7RU, Tyne & Wear, England
[4] Royal Free Hosp, Dept Endocrinol, London NW3 2QG, England