Worsening of Congenital Hypothyroidism After Start of Carob-bean Gum Thickened Formula: Is There a Link? A Case Report

Congenital hypothyroidism (CH), if not correctly treated with levothyroxine (L-T4), may cause permanent intellectual disability. If patients treated with L-T4 do not achieve good thyroid stimulating hormone (TSH) control, the possibility of poor compliance and/or poor absorption of L-T4 should be investigated. We describe an infant with CH whose thyroid hormone levels worsened after she started a carob-bean gum thickened formula. A baby girl was diagnosed with CH by newborn screening [confirmatory venous blood TSH was 496.0 μIU/mL and free thyroxine (fT4) was 0.13 ng/dL]. Five weeks after beginning replacement L-T4 treatment (10.6 μg/kg per day), thyroid function normalized (TSH 2.72 μIU/mL, fT4 2.08 ng/dL). However, five weeks later, thyroid function test results had worsened (TSH 31.1 μIU/mL, fT4 1.27 ng/dl), which worsened further (TSH 44.8 μIU/mL, fT4 1.16 ng/dL) even after L-T4 dose was increased (10.9 μg/kg per day). Anamnesis disclosed that she had started a carob-bean gum thickened formula to combat gastroesophageal reflux disease rather than regular type 1 formula milk. The anti-reflux milk formula was discontinued. Fourteen days later her TSH level dropped to 0.38 μIU/mL and fT4 increased to 2.68 ng/dL, allowing the L-T4 dose to be reduced (from 10.9 μg/kg per day to 8.0 μg/kg per day). These findings suggest that carob-bean gum thickened formula may affect absorption of L-T4. If such formulas are used, we recommend a more frequent evaluation of thyroid function. In CH infants, inexplicably high TSH levels may be caused by gastrointestinal disorders or the interference from drugs or other substances, including some types of milk formula, which impair L-T4 absorption.