Iron oral (salts: sulfate, bisglycinate, fumarate, gluconate, polymaltose; heme iron polypeptide, lactoferrin) × Levothyroxine sodium
MajorOral iron preparations; antianaemic agents×Thyroid hormones
Updated · Methodology
Mechanism
Iron forms a stable, poorly soluble complex with levothyroxine. The AEMPS label lists iron supplements among substances that reduce absorption of the hormone, and NIH ODS names levothyroxine among the drugs iron interacts with. The pairing is common: iron deficiency anaemia and hypothyroidism often occur in the same patient.
Symptoms
TSH rises on an unchanged dose, with fatigue and cold intolerance the patient puts down to the anaemia. The overlap misleads: both conditions cause tiredness.
Management
Levothyroxine in the morning on an empty stomach, iron after a meal or in the evening, 4–5 hours apart. Iron works better with vitamin C and away from dairy. When iron is started in a patient stable on levothyroxine, repeat TSH after 6–8 weeks: the dose may need adjusting for as long as the iron course lasts.
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Sources
- AEMPS: Ficha técnica Levotiroxina Aristo 100 microgramos comprimidos EFG (2023)– AEMPS, CIMA, nº registro 84484, sección 4.5
- NIH Office of Dietary Supplements: Iron – Health Professional Fact Sheet (2025)– NIH ODS, Iron, Interactions with Medications