A 52-year-old female with hypothyroidism and bipolar I disorder takes levothyroxine 125 mcg each morning. She has been recently stabilized on lithium 900 mg daily and quetiapine 300 mg at bedtime, with a most recent TSH of 2.1 mIU/L. At a follow-up visit, her TSH has risen to 8.7 mIU/L despite no changes to her levothyroxine dose or brand. Upon detailed medication review, she reports that she recently began taking calcium carbonate 1000 mg and a multivitamin containing iron at the same time as her levothyroxine each morning. Which of the following best explains the mechanism of this drug interaction?