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Background: Hypothyroidism and iron deficiency are commonly occurring states, particularly in older patients. Co-morbidity of these conditions is frequent. In isolation, timing of iron or levothyroxine replacement is unimportant. However, if given simultaneously, malabsorption of levothyroxine can result (Liwanpo L, Hershman JM, Best Pract Res Clin Endocrinol Metab. 2009;23(6):781-92). This may lead to hypothyroidism on commencement of iron replacement, dose alteration of levothyroxine and potential thyrotoxicosis on cessation of oral iron (Campbell NR et al, Ann Intern Med 1992;117:1010-3, Leger CS, Ooi TC, Endocrinologist 1999;9:493-5). The British National Formulary (BNF) recommends a minimum interval of two hours between levothyroxine and later ferrous salt administration.

Our aim was to assess whether levothyroxine and ferrous salts were prescribed according to BNF recommendation.

Sampling methods: A cross-sectional survey of medical inpatients was undertaken at a district general hospital to assess whether levothyroxine and ferrous salts were prescribed according to BNF recommendation. An expectation of 100% concordance with BNF recommendation was set.

Results: Data was collected from 291 inpatients, 5.5% of these patients were prescribed levothyroxine and a ferrous preparation. The mean age was 81 (44-101). 1/16 (6.3%) of patients were prescribed levothyroxine and a ferrous preparation with an adequate time gap.

Conclusions: Co-prescription of levothyroxine and ferrous salts was poorly adherent to recommendations creating potential for under-replacement of levothyroxine and clinically significant loss of euthyroid status in some patients. The mean age of patients requiring both drugs was in the geriatric range, a group more likely to suffer morbidity when experiencing a dysthyroid state. Errors at prescription were not corrected on medicines reconciliation. After raising awareness of the clinical problem at a local clinical governance meeting, we have devised an education programme directed at junior pharmacists and doctors in order to increase awareness of this well-established but little-known drug interaction reducing this simple prescription error.

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