The short answer: Levothyroxine is absorbed in your gut, but only part of each dose is actually absorbed. Coffee, calcium, iron, fibre, soy and low stomach acid can shrink that part. Take your tablet on an empty stomach with water, 30 to 60 minutes before breakfast, and leave at least four hours between it and any mineral supplements. If anything changes in your medication or your diet, have your TSH checked after six to eight weeks. Only your doctor should change the dose.
Why the way you take levothyroxine matters so much
Levothyroxine replaces the hormone T4, which your thyroid no longer makes in sufficient amounts when you have Hashimoto’s. The tablet is absorbed mainly in the small intestine, and absorption is best when your stomach and gut are empty and acidic. Even small changes in absorption shift your TSH. Many women whose results go up and down on the same dose don’t have a dose problem. They have a problem with how they take it.
The American Thyroid Association recommends taking levothyroxine 60 minutes before breakfast, or in the evening at least three hours after your last meal, always the same way and separated from foods and medicines that interfere with absorption.[1] In a randomised trial, taking it in the evening worked just as well, or even slightly better.[2] What matters is that you stick to one routine.
What gets in the way of absorption in everyday life
Coffee
Coffee is the most common culprit. When levothyroxine was swallowed with espresso, absorption tests showed the body took up 27 to 36 per cent less than with water. When the coffee was only drunk 60 minutes after the tablet, there was no difference.[3] Green and black tea aren’t ideal either. Water is the only drink that belongs with your tablet.
Calcium and calcium-rich foods
In a study of 20 patients, TSH rose on average from 1.6 to 2.7 mIU/l when calcium carbonate was taken with the tablet, and fell again once it was stopped.[4] Calcium binds levothyroxine in the gut. That applies to supplements, but also to calcium-rich meals and calcium-fortified plant drinks.
Iron
Iron forms poorly soluble compounds with levothyroxine. In a study of 14 patients, TSH rose on average from 1.6 to 5.4 mIU/l after twelve weeks of taking ferrous sulphate with the tablet.[5] This matters especially in Hashimoto’s with low iron stores: iron supplements are fine if you need them, just not at the same time as your tablet.
Magnesium and aluminium
Antacids containing aluminium reduce absorption. For magnesium, the evidence for an effect is weaker. A specialist review in the German pharmacists’ journal Pharmazeutische Zeitung advises leaving at least two, ideally four, hours between levothyroxine and mineral supplements like these.[6]
Fibre, psyllium and soy
Many types of fibre bind levothyroxine. In one study, people on a high-fibre diet needed a higher dose and absorbed less.[7] Fibre is still important for your gut. It just doesn’t belong right next to your tablet. Psyllium husk is best taken four hours apart from your tablet.
Soy can increase the dose you need if you eat it close to taking your tablet.[8] In laboratory and animal studies, soy isoflavones also inhibited the enzyme TPO (thyroid peroxidase, the enzyme your thyroid needs to make its hormones).[9] That’s why in my coaching I leave soy out in Hashimoto’s.
Lactose
Many tablets contain lactose as a filler. If you are lactose intolerant, disturbed digestion can reduce absorption: in one study, TSH fell after eight weeks of a low-lactose diet in people with Hashimoto’s and lactose intolerance.[10] Lactose-free products exist. Talk to your doctor about switching.
Medicines that interact with levothyroxine
| Medicine or remedy | What happens | What you can do |
|---|---|---|
| Acid-suppressing medication (e.g. omeprazole, pantoprazole) | Less stomach acid, poorer absorption; in one study the dose needed rose by 22 to 37 per cent[11] | Leaving a gap doesn’t solve this. Have your TSH checked 6–8 weeks after starting or stopping |
| Calcium, iron, magnesium and aluminium preparations | Bind levothyroxine in the gut[4,5,6] | At least 4 hours apart |
| Cholesterol-lowering drugs colestyramine and colestipol | Bind levothyroxine strongly[12] | 4 to 5 hours apart |
| Oestrogens (the pill, hormone replacement therapy) | More transport protein in the blood, less free hormone; in one study the dose needed rose in 7 of 11 women[13] | Have your TSH checked 12 weeks after starting or switching |
| Metformin | Lowers TSH in hypothyroidism; in a meta-analysis of people on levothyroxine, by just over 1 mIU/l on average; fT4 usually stays the same[15] | Don’t judge TSH on its own; ask for fT4 and fT3 as well |
| St John’s wort | In a small case-control study, raised TSH was more common with St John’s wort; the link was not statistically significant.[16] In my coaching I see the effect clearly | Tell your doctor you take it; have your TSH checked 6–8 weeks after starting |
| Lithium, amiodarone | Can themselves trigger an underactive or overactive thyroid[17] | Have your thyroid levels checked regularly by your doctor |
| Coumarin-type blood thinners (e.g. warfarin, phenprocoumon) | Thyroid hormone can strengthen their effect[12] | Have your clotting checked more closely after a dose change |
| Blood-sugar-lowering medicines | Thyroid hormone can weaken their blood-sugar-lowering effect[12] | Keep an eye on your blood sugar after a dose change |
| Sertraline | TSH can rise[12] | Have your TSH checked after starting |
| Corticosteroids and beta-blockers | Can slow the conversion of T4 into T3[12] | Ask for fT3 to be tested as well |
The patient information leaflet for your medicine is the definitive guide to interactions and timings. Never stop or switch medicines or herbal remedies on your own.
My everyday plan for taking levothyroxine
| Time (example) | What |
|---|---|
| 6:30 | Levothyroxine with a glass of water |
| 7:15 | Breakfast with protein, coffee not before this |
| 11:00 | Iron, calcium or magnesium, if prescribed |
| Lunch and dinner | Fibre, psyllium and pulses |
This keeps all the necessary gaps, and the nutrients you often need with Hashimoto’s still get where they need to go.
Before your blood test
For a meaningful result, don’t take your tablet before the blood test that morning; take it straight afterwards. In the hours after you take it, fT4 rises markedly; in a small study it was around 40 per cent above the starting value three to four hours later.[18] Try to have your levels tested at the same time of day every time. If you take high-dose biotin, for example in hair, nail or collagen supplements, tell your doctor beforehand: from about 5 mg a day it can distort thyroid tests.[14,19] You’ll find everything about the values and planning your blood test in “Understanding your thyroid blood tests”.
Patterns I often see in my coaching
| Pattern | Typical signs | Where I start |
|---|---|---|
| TSH fluctuates on the same dose | Coffee shortly after the tablet, taking it at different times | Set a fixed routine, coffee 60 minutes later |
| Dose needed rises for no obvious reason | New acid-suppressing medication, new pill, St John’s wort, iron in the morning | Go through the medication list with your doctor, check TSH |
| Tablet is right, energy isn’t | TSH normal, fT3 in the lower third | Check selenium, iron and the stress axis |
| Unsettled gut, unstable results | Bloating after milk, lactose in the tablet | Check for lactose intolerance, build up gut health |
Patterns are signs from my coaching experience, not diagnoses.
Where to start
- My free thyroid calculator shows you how your results relate to one another.
- Your free clarity call: 20 to 30 minutes on Zoom. Together we look at where you stand and which next step suits you.
Further reading: “Understanding your thyroid blood tests”, “Diet and Hashimoto’s” and “Always tired? These blood tests are worth checking”.
Frequently asked questions
How long after taking levothyroxine can I drink coffee?
At least 60 minutes. When levothyroxine was swallowed with coffee, absorption fell by up to a third in studies; when the coffee was drunk an hour later, there was no difference.[3]
Can I take levothyroxine at night?
Yes, as long as your last meal was at least three hours earlier.[1] In a randomised trial, taking it at bedtime worked just as well.[2] Stick to one routine and discuss any switch with your doctor.
How long should I wait between levothyroxine and iron or calcium?
At least four hours. Both bind levothyroxine in the gut and can make your TSH rise considerably.[4,5]
Does leaving a gap help with acid-suppressing medication such as omeprazole?
No. They work all day long. Have your TSH checked six to eight weeks after starting or stopping them.[11]
Does the pill change how much levothyroxine I need?
It can. Oestrogens increase the transport protein in your blood, and the dose you need may rise.[13] Have your TSH checked about twelve weeks after starting or switching.
Should I take levothyroxine before a blood test?
No. On the morning of the test, take your tablet only afterwards, otherwise the lab will pick up a temporarily raised fT4.[18]
Read on
- Understanding your thyroid blood tests
- Diet and Hashimoto’s
- Always tired? These blood tests are worth checking
This article does not replace medical advice. Diagnoses are made by your doctor; changes to medication belong in medical hands.
Sources
- Jonklaas J et al. Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association task force on thyroid hormone replacement. Thyroid 2014;24:1670–1751. doi.org/10.1089/thy.2014.0028 · Guideline (USA)
- Bolk N et al. Effects of evening vs morning levothyroxine intake: a randomized double-blind crossover trial. Arch Intern Med 2010;170:1996–2003. doi.org/10.1001/archinternmed.2010.436 · Double-blind RCT (n = 90)
- Benvenga S et al. Altered intestinal absorption of L-thyroxine caused by coffee. Thyroid 2008;18:293–301. doi.org/10.1089/thy.2007.0222 · Case series with absorption tests
- Singh N, Singh PN, Hershman JM. Effect of calcium carbonate on the absorption of levothyroxine. JAMA 2000;283:2822–2825. doi.org/10.1001/jama.283.21.2822 · Prospective cohort study (n = 20)
- Campbell NR et al. Ferrous sulfate reduces thyroxine efficacy in patients with hypothyroidism. Ann Intern Med 1992;117:1010–1013. doi.org/10.7326/0003-4819-117-12-1010 · Intervention study (n = 14)
- Gerdemann A, Griese N, Schulz M. Interaktionen: L-Thyroxin [Interactions: levothyroxine]. Pharmazeutische Zeitung, 06.11.2007. pharmazeutische-zeitung.de · Specialist review
- Liel Y, Harman-Boehm I, Shany S. Evidence for a clinically important adverse effect of fiber-enriched diet on the bioavailability of levothyroxine in adult hypothyroid patients. J Clin Endocrinol Metab 1996;81:857–859. doi.org/10.1210/jcem.81.2.8636317 · Case series
- Messina M, Redmond G. Effects of soy protein and soybean isoflavones on thyroid function in healthy adults and hypothyroid patients: a review of the relevant literature. Thyroid 2006;16:249–258. doi.org/10.1089/thy.2006.16.249 · Review
- Doerge DR, Sheehan DM. Goitrogenic and estrogenic activity of soy isoflavones. Environ Health Perspect 2002;110 Suppl 3:349–353. doi.org/10.1289/ehp.02110s3349 · Review (laboratory and animal data)
- Asik M et al. Decrease in TSH levels after lactose restriction in Hashimoto’s thyroiditis patients with lactose intolerance. Endocrine 2014;46:279–284. doi.org/10.1007/s12020-013-0065-1 · Intervention study without placebo control (n = 83)
- Centanni M et al. Thyroxine in goiter, Helicobacter pylori infection, and chronic gastritis. N Engl J Med 2006;354:1787–1795. doi.org/10.1056/NEJMoa043903 · Cohort study
- Gebrauchsinformation eines levothyroxinhaltigen Fertigarzneimittels [Package leaflet of a levothyroxine-containing medicinal product], version 12/2010 · Package leaflet
- Arafah BM. Increased need for thyroxine in women with hypothyroidism during estrogen therapy. N Engl J Med 2001;344:1743–1749. doi.org/10.1056/NEJM200106073442302 · Prospective study
- Bhimireddy N, Na S, Cho E. The implications of biotin supplementation on multiple sclerosis, hair loss conditions, and thyroid disorders. Mult Scler Relat Disord 2026;116:107974. doi.org/10.1016/j.msard.2026.107974 · Review
- Lupoli R et al. Effects of treatment with metformin on TSH levels: a meta-analysis of literature studies. J Clin Endocrinol Metab 2014;99:E143–E148. doi.org/10.1210/jc.2013-2965 · Meta-analysis (7 datasets, n = 206)
- Ferko N, Levine MA. Evaluation of the association between St. John’s wort and elevated thyroid-stimulating hormone. Pharmacotherapy 2001;21:1574–1578. doi.org/10.1592/phco.21.20.1574.34483 · Case-control study (n = 74)
- Burch HB. Drug effects on the thyroid. N Engl J Med 2019;381:749–761. doi.org/10.1056/NEJMra1901214 · Review
- Marina M et al. Circulating concentrations of free thyroxine after an oral intake of liquid LT4 taken either during fasting conditions or at breakfast. Acta Biomed 2016;87:247–252. ncbi.nlm.nih.gov/pmc/articles/PMC10521879 · Comparative study (n = 21)
- Li D et al. AACC Guidance Document on Biotin Interference in Laboratory Tests. J Appl Lab Med 2020;5:575–587. doi.org/10.1093/jalm/jfz010 · Guidance document (laboratory medicine society)
