Interaction fact sheet
Levothyroxine, calcium and iron
Take separately: calcium and iron interfere with the absorption of thyroid medication.
When taken at the same time, calcium and iron bind to levothyroxine in the gut. In two trials, TSH increased, a sign that treatment had become less effective. Take levothyroxine on an empty stomach and these supplements several hours later.
In brief
- Mechanism
- Calcium and iron bind to levothyroxine in the intestine and prevent it from being absorbed.
- Magnitude
- TSH rose from 1.6 to 2.7 with calcium, and from 1.6 to 5.4 with iron, in two trials.
- Who is affected
- People being treated for hypothyroidism, particularly after age 50, when calcium and iron are often prescribed.
- What to do
- Levothyroxine on an empty stomach, then calcium or iron at least 4 hours later. Have your TSH checked if you start a supplement.
With calcium
In a trial published in JAMA, 20 patients taking levothyroxine took 1,200 mg of calcium as calcium carbonate for 3 months, at the same time as their medication. Their free T4 fell and their average TSH rose from 1.6 to 2.7 mIU/L. One in five patients exceeded the normal range. Everything returned to normal after calcium was stopped.
With iron
In 14 patients with hypothyroidism, 300 mg of ferrous sulfate taken with levothyroxine for 12 weeks raised average TSH from 1.6 to 5.4 mIU/L. Nine of them developed more signs of hypothyroidism. When mixed in the laboratory, iron and thyroxine form a poorly soluble complex.
Other products concerned
The same problem occurs with magnesium, certain stomach-coating medicines, acid-reducing medicines and, to a lesser extent, coffee drunk immediately after the tablet. Calcium-rich dairy products eaten at breakfast can also play a role.
In practice
What you can do
- Take levothyroxine in the morning on an empty stomach, with water, about 30 minutes before breakfast.
- Leave at least 4 hours between levothyroxine and calcium, iron or magnesium.
- Keep your dosing times consistent from day to day.
When to seek advice
- Before starting or stopping a calcium, iron or magnesium supplement: a TSH check may be useful.
- Fatigue, feeling cold or weight gain returning while on treatment.
- Pregnancy: levothyroxine requirements change.
Main sources
Frequently Asked Questions (FAQ)
How many hours between levothyroxine and iron?
At least 4 hours, taking levothyroxine first, in the morning on an empty stomach. Iron can be taken in the evening or mid-afternoon.
Does milk at breakfast interfere with levothyroxine?
Calcium in milk can slightly reduce its absorption. Waiting around 30 minutes after the tablet before eating limits the effect.
What about magnesium?
It acts like calcium: leave several hours between doses. Stomach-coating medicines and antacids containing it are also affected.