Liothyronine and Levothyroxine Prescribing in England: A Comprehensive Survey and Evaluation.

INTRODUCTION The approach to thyroid hormone replacement varies across centres but the extent and determinants of variation is unclear. We evaluated geographical variation in levothyroxine (LT4) and liothyronine (LT3) prescribing across General Practices in England and analysed the relationship of prescribing patterns to clinical and socioeconomic factors. METHODS Data was downloaded from the NHS monthly General Practice Prescribing Data in England for the period 2011-2020. RESULTS The study covered a population of 19.4 million women over 30 years of age, attending 6,660 GP practices and being provided with 33.7 million prescriptions of LT4 and LT3 at a total cost of £90million / year. Overall, 0.5% of levothyroxine treated patients continue to receive liothyronine. All Clinical Commission Groups (CCGs) in England continue to have at least one liothyronine prescribing practice and 48.5% of English general practices prescribed liothyronine in 2019-20. Factors strongly influencing more levothyroxine prescribing (model accounted for 62% of variance) were the CCG to which the practice belonged and the proportion of people with diabetes registered on the practice list plus antidepressant prescribing, with socioeconomic disadvantage associated with less levothyroxine prescribing. For liothyronine prescribing (model accounted for 17% of variance), factors that were associated with increased levels of liothyronine prescribing were antidepressant prescribing and % of type 2 diabetes mellitus individuals achieving HbA1c control of 58mmol/mol or less. Factors that were associated with reduced levels of liothyronine prescribing included smoking and higher obesity rates. CONCLUSION In spite of strenuous attempts to limit prescribing of liothyronine in general practice a significant number of patients continue to receive this therapy, although there is significant geographical variation in the prescribing of this as for levothyroxine, with specific general practice and CCG related factors influencing prescribing of both levothyroxine and liothyronine across England.