Levothyroxine with calcium, iron, magnesium or coffee: why timing matters

By Milen Cholakov, MPharm · 26 September 2026 · 4 min read

Levothyroxine replaces thyroid hormone in people with an underactive thyroid. It is usually taken once a day, often for life, and the dose is fine-tuned with blood tests [3]. That makes it sensitive to anything that changes how much of each tablet you absorb. Some of the most common culprits are ordinary: calcium, iron and magnesium supplements, and the morning coffee.

Why absorption is easily disturbed

Levothyroxine is absorbed from the gut incompletely and variably [1]. It is better absorbed on an empty stomach than before or after a meal, which is why product information asks you to take it in the morning, at least half an hour before breakfast [2]. The NHS advises taking it 30 to 60 minutes before your first meal of the day, before any drink with caffeine such as tea or coffee, and before any other medicine, at the same time each day [3].

Why small changes matter

Your dose is set by your blood tests. A daily habit that quietly reduces absorption can make the dose look too low, and dropping that habit later can have the opposite effect. Being on the wrong dose can cause side effects such as feeling sick, diarrhoea, headaches or problems sleeping [3]. The SmPC also stresses regular monitoring of TSH during long-term treatment, because undetected overtreatment is linked to atrial fibrillation and fractures in older patients [1].

Calcium, iron and magnesium: what the product information says

Official product information for levothyroxine lists these minerals among the things that reduce its absorption, and asks you to space them apart. The exact interval differs between products:

• A UK Summary of Product Characteristics (SmPC) states that absorption is possibly reduced by antacids, calcium salts, oral iron, sucralfate and some cholesterol-lowering resins, and that administration should be separated by 4–5 hours [1].
• The Irish patient leaflet for another brand advises taking the tablet at least 2 hours before calcium, magnesium, aluminium or iron supplements, antacids, sucralfate and proton pump inhibitors, and at least 4 hours before medicines such as cholestyramine, colestipol and orlistat [2].

So there isn't one universal number. Follow the leaflet of the product you actually take, and keep your routine the same from day to day, because your dose was adjusted to that routine. These minerals don't only come as single supplements: check multivitamins, calcium with vitamin D, iron tablets, magnesium products and indigestion remedies as well.

Coffee

Coffee can reduce levothyroxine absorption too. Italian researchers described eight women whose thyroid blood tests were higher than expected while they swallowed their tablet with coffee or espresso [4]. In absorption tests, taking the tablet with coffee instead of water reduced the amount absorbed (area under the curve) by 36% in patients and 27% in volunteers. When the tablet was taken with water and coffee followed 60 minutes later, absorption was similar to water alone [4]. This fits the NHS advice to wait 30 to 60 minutes before tea or coffee [3].

Other things that can affect levothyroxine

• Food: soya products and high-fibre foods may affect absorption, and your doctor may need to adjust the dose [2].
• Stomach-acid medicines: proton pump inhibitors may reduce absorption. The SmPC recommends monitoring when they are used together, and also when the acid medicine is stopped [1].
• Faster breakdown: rifampicin, carbamazepine, phenytoin and St John's wort can speed up levothyroxine's metabolism [1]. (See: St John's wort and medicines.)
• Hormones: oestrogen, including hormone replacement therapy and oral contraceptives, may increase the dose needed [1].
• Biotin (vitamin B7): it doesn't change levothyroxine itself, but it can interfere with thyroid blood tests and give falsely high or low results. Tell the laboratory if you take it [1, 3].

When to talk to your doctor or pharmacist

• Before adding a calcium, iron or magnesium supplement, a multivitamin, an indigestion remedy or biotin.
• If your thyroid blood tests change without an obvious reason, or symptoms such as tiredness, weight gain or low mood come back [3].
• If you are switched to a different levothyroxine product: a small number of people report side effects after changing products [1].
• If you are pregnant or planning a pregnancy, as you may need more frequent blood tests [3].
• If the timing is hard to fit into your day. A pharmacist can help you plan a routine you can keep.

Don't change your levothyroxine dose on your own. Any change should be guided by your doctor and your blood tests.

This article is general information, not medical advice, diagnosis or treatment. Don't start, stop or change a medicine without talking to your doctor or pharmacist.