Levothyroxine absorption falls when it is taken with calcium, iron, antacids, sucralfate, bile acid sequestrants or phosphate binders, and is reduced by food and by drugs that raise gastric pH such as proton pump inhibitors.
A TSH that rises without explanation in a patient on a stable dose is often an absorption problem, not a need for more hormone. Asking about new supplements and how the tablet is taken usually finds the cause before the dose is increased.
- Advise taking levothyroxine on an empty stomach, 30 to 60 minutes before breakfast, or consistently at bedtime.
- Separate it by about four hours from calcium, iron, antacids and phosphate binders.
- Ask about new supplements or a proton pump inhibitor when TSH rises on a stable dose.
- Recheck TSH six to eight weeks after starting or stopping an interacting drug.
Why it matters
Dose increases for an interaction leave patients over-replaced when the interacting drug stops.
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