Levothyroxine is a synthetic form of the thyroid hormone (T4) prescribed daily to treat hypothyroidism, a condition where the thyroid gland does not produce sufficient hormone. This medication maintains the body’s metabolism and energy levels. Magnesium is a common over-the-counter mineral supplement used to support muscle function, nerve health, and sleep quality. Because many supplements interfere with prescription medications, people often wonder if they can take magnesium concurrently with levothyroxine. Taking magnesium too close to levothyroxine can significantly decrease the effectiveness of the thyroid medication.
How Levothyroxine Is Absorbed
The body’s ability to absorb levothyroxine is highly sensitive and depends on specific conditions within the digestive tract. Levothyroxine must dissolve and then be transported across the intestinal lining to enter the bloodstream. The majority of this absorption occurs in the small intestine, primarily within the jejunum and upper ileum.
Absorption is generally incomplete, with only about 40% to 80% of an oral dose successfully entering the body. To maximize absorption, the medication should be taken on an empty stomach to ensure a consistent amount of the hormone is absorbed. Any substance in the gastrointestinal tract that interferes with the drug’s dissolution or transport can reduce its bioavailability, leading to fluctuating levels of thyroid hormone in the blood.
Why Magnesium Causes Reduced Absorption
The primary issue with taking magnesium and levothyroxine together involves a physical interaction in the digestive tract. Magnesium is a divalent cation, meaning it is a positively charged ion. In the stomach and small intestine, this charged ion can physically bind to the levothyroxine molecule.
This binding process, known as complexation, creates an insoluble compound. This new complex is too large or chemically unsuitable to pass through the intestinal wall and into the bloodstream. Instead, the bound levothyroxine is excreted from the body as waste, effectively reducing the amount of medication available to the patient.
When this interaction occurs, the bioavailability of the thyroid hormone is lowered, disrupting the thyroid system. Taking magnesium supplements simultaneously with levothyroxine can cause an increase in Thyroid-Stimulating Hormone (TSH) levels. An elevated TSH reading indicates the body is not receiving enough thyroid hormone, signaling the medication is not working as intended. This mechanism is also observed with other polyvalent cations, such as iron, calcium, and aluminum, which significantly reduce levothyroxine absorption.
Guidelines for Safe Supplement Timing
To ensure the full dose of levothyroxine is absorbed, the timing of the medication and the magnesium supplement must be separated. The standard clinical recommendation is to maintain a minimum separation of at least four hours between taking levothyroxine and ingesting magnesium. This interval allows the levothyroxine to dissolve and pass through the small intestine’s primary absorption sites before the magnesium enters the digestive system.
For optimal consistency, levothyroxine should be taken first thing in the morning with plain water, ideally 30 to 60 minutes before eating breakfast. Magnesium supplements can then be taken later in the day, such as with lunch, dinner, or at bedtime. This strategy prioritizes the absorption of the thyroid medication, which requires stable blood levels.
This separation rule applies to all forms of magnesium, including common supplements like citrate, oxide, or glycinate, as well as mineral-containing multivitamins and some antacids. After starting any new supplement, patients should have their TSH levels monitored by their healthcare provider within six to eight weeks. If the addition of magnesium alters thyroid hormone levels, the levothyroxine dosage may need adjustment. Any changes to a medication or supplement regimen should always be discussed with a doctor.