Summary

Ferrous sulfate significantly reduces levothyroxine absorption when taken together, potentially leading to inadequate thyroid hormone replacement. This interaction requires careful timing of medication administration to maintain therapeutic effectiveness.

Introduction

Levothyroxine is a synthetic thyroid hormone (T4) used to treat hypothyroidism and thyroid hormone deficiency. It belongs to the thyroid hormone class and is essential for maintaining normal metabolic function. Ferrous sulfate is an iron supplement commonly prescribed to treat or prevent iron deficiency anemia. It provides elemental iron necessary for red blood cell production and oxygen transport throughout the body.

Mechanism of Interaction

Ferrous sulfate interferes with levothyroxine absorption in the gastrointestinal tract through chelation and binding mechanisms. Iron forms insoluble complexes with levothyroxine in the acidic stomach environment, preventing the thyroid hormone from being properly absorbed in the small intestine. This chelation process can reduce levothyroxine bioavailability by up to 64%, significantly compromising the medication's therapeutic effectiveness.

Risks and Symptoms

The primary risk of this interaction is reduced levothyroxine effectiveness, which can lead to persistent or worsening hypothyroid symptoms including fatigue, weight gain, cold intolerance, and depression. Inadequate thyroid hormone replacement may result in elevated TSH levels, requiring dose adjustments. In severe cases, untreated hypothyroidism can lead to cardiovascular complications, cognitive impairment, and myxedema. Patients may experience breakthrough hypothyroid symptoms despite taking their prescribed levothyroxine dose.

Management and Precautions

Separate administration of levothyroxine and ferrous sulfate by at least 4 hours to minimize interaction. Take levothyroxine on an empty stomach, typically 30-60 minutes before breakfast, and schedule iron supplements later in the day. Monitor thyroid function tests (TSH, free T4) more frequently when initiating or discontinuing iron supplementation. Consider dose adjustments of levothyroxine if TSH levels become elevated. Educate patients about proper timing and the importance of consistent medication schedules. Alternative iron formulations or intravenous iron may be considered in cases where oral separation is impractical.

Levothyroxine interactions with food and lifestyle

Levothyroxine absorption can be significantly reduced when taken with food, particularly high-fiber foods, soy products, and calcium-rich foods. Coffee may also decrease absorption. Take levothyroxine on an empty stomach, at least 30-60 minutes before breakfast or at bedtime (at least 3 hours after the last meal). Avoid taking levothyroxine with calcium supplements, iron supplements, or antacids containing aluminum, magnesium, or calcium within 4 hours of dosing. Maintain consistent timing of administration and dietary habits to ensure stable thyroid hormone levels.

Ferrous sulfate interactions with food and lifestyle

Ferrous sulfate absorption is significantly reduced when taken with calcium-rich foods (dairy products), coffee, tea, or foods high in fiber. Take ferrous sulfate on an empty stomach or with vitamin C to enhance absorption. Avoid taking within 2 hours of consuming dairy products, coffee, tea, or high-fiber foods. Antacids and proton pump inhibitors can also reduce iron absorption.