Dentifrice Reducing Levothyroxine Efficacy in Children
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To the Editor: Absorption of thyroxine supplement, used for the treatment of hypothyroidism, is interfered by several drugs and food. A two-years-old girl, a diagnosed case of congenital hypothyroidism (CH) presented to us with elevated thyroid stimulating hormone (TSH) (38 mIU/l). She was started on L-thyroxine (Levothyroxine) at a dose 15 mcg/kg/d elsewhere from day 5 of life after being diagnosed as CH on neonatal screening and was on regular follow-up since then. Previous TSH levels were found be within normal range on multiple occasions as per the medical records and she was taking L-thyroxine 50 mcg/d before visiting us. Growth and developmental milestones were appropriate for her age. She had weight of 12 kg (0 to +1SD) and height of 84 cm (0 to +1 SD) at presentation. Compliance was good. Initially thyroxine dose was increased and compliance was ensured. Despite of hike in dose, repeat TSH after 1 mo remained to be elevated (42 mIU/l). There was no history suggestive of malabsorption syndrome and any other drug intake. On taking detailed history, parents revealed that the child had started brushing her teeth for last few months and she used to swallow the tooth paste while brushing. They used to give thyroxine immediately after brushing in empty stomach. Possibility of toothpaste hampering the absorption of thyroxine was considered. Hence, we advised to change the brushing pattern and introduced reward technique for the child and advised to give the medication in same dose 30–45 min after brushing. TSH level became normal (TSH 3.26 mIU/l) subsequently and we were able to taper down the dose of thyroxine to 37.5 mcg/d on follow-up.
Drugs like ferrous sulfate, bile acid sequestrants and aluminum hydroxide decrease the absorption of levothyroxine [1, 2, 3]. In addition, high-fiber diets and other foods impair levothyroxine absorption too . As per our knowledge, toothpaste interfering with absorption of thyroxine has not been reported previously. Most of the common Indian toothpastes available in the market contain calcium carbonate or aluminum hydroxide as abrasive, sorbitol or glycerin as humectant and sodium lauryl sulfate as detergent. Flavourants are used to increase consumer acceptance too. A study of 20 patients receiving long-term levothyroxine replacement therapy indicates that calcium carbonate reduces T4 absorption and increases serum thyrotropin levels . Uniqueness of our case lies in the fact that, though the parents were aware of taking medication in empty stomach but ignored the factor of tooth paste swallowing. Calcium containing toothpaste interfering with absorption of thyroxine is a less known fact. It is important for patients, parents and health care providers to have knowledge of various drugs and substances that interfere with LT4 absorption.
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