Levothyroxine & Iron Timing Calculator
This tool helps you calculate the safe time interval between taking levothyroxine (thyroid medication) and iron supplements. Following the 4-hour separation rule is critical to ensure your thyroid medication works properly.
How to Use This Calculator
Enter when you take one of your medications and we'll calculate the earliest safe time to take the other.
The 4-hour separation rule is based on scientific evidence. Taking levothyroxine and iron within 4 hours of each other can reduce absorption by up to 39%, causing symptoms to return.
Choose the approach that works best for your daily routine:
Option 1 (Morning Levothyroxine): Take levothyroxine first thing in the morning, then wait at least 4 hours before taking iron. This works well if you take iron with lunch.
Option 2 (Morning Iron): Take iron with breakfast, then wait at least 4 hours before taking levothyroxine. Best if you take levothyroxine at bedtime.
If you're taking levothyroxine for hypothyroidism and also need iron supplements for anemia, you're not alone. Millions of people manage both. But hereâs the problem: iron and levothyroxine bind together in your gut, turning your thyroid medication into something your body canât absorb. This isnât a minor issue. It can cause your TSH levels to spike, your energy to crash, and your symptoms to return-despite taking your pills exactly as prescribed.
Why Iron Ruins Levothyroxine Absorption
Levothyroxine works by replacing the thyroid hormone your body doesnât make enough of. Itâs absorbed in the upper part of your small intestine, where the environment needs to be acidic and empty. Iron supplements, especially ferrous sulfate (the most common type), contain charged metal ions that latch onto levothyroxine molecules like magnets. This creates a thick, insoluble complex that just passes through your system-unabsorbed.Studies show this interaction can slash levothyroxine absorption by up to 39%. Thatâs not a small drop. Itâs enough to push your TSH from a normal 2.0 to a concerning 7.0 in just a few weeks. You might feel fine at first, but over time, fatigue, weight gain, cold hands, and brain fog creep back in-not because your thyroid is failing again, but because your medication isnât working.
This isnât new science. The first paper on this interaction came out in 1976. Since then, dozens of studies have confirmed it. Even the FDA updated levothyroxine labeling in 2022 to make the warning clearer. Yet, a 2025 audit in the UK found that 84% of patients taking both medications were still taking them at the same time.
The 4-Hour Rule: What the Experts Say
The consensus among endocrinologists and major medical organizations is clear: separate iron and levothyroxine by at least four hours.- The British National Formulary (BNF 2024) says: âSeparate administration by at least four hours.â
- NICE guidelines (NG145, 2023) specifically mention ferrous sulfate and require a four-hour gap.
- AbbVieâs Synthroid prescribing information (January 2024) states: âTake at least four hours before or after iron supplements.â
- MedlinePlus (NIH) and Thyroid UK both align with the four-hour standard.
Some sources, like Thyroid UKâs patient guide, mention a two-hour window as âsometimes acceptable.â But thatâs not the rule-itâs the exception. The four-hour gap exists because peopleâs digestion varies. Someone with celiac disease, IBS, or slow gut motility might absorb levothyroxine even slower. A two-hour gap might work for one person and fail for another. The four-hour rule gives you a safety buffer.
Hereâs what happens when you follow it: a 2022 study of 150 patients showed that those who spaced their doses by four hours or more maintained normal TSH levels 89% of the time. Those who didnât? Only 62% stayed in range.
When and How to Take Them
The goal is to keep levothyroxine on an empty stomach and iron away from it. There are two proven strategies:Strategy 1: Morning Levothyroxine, Afternoon Iron
- Take levothyroxine first thing in the morning, with a full glass of water, 30-60 minutes before breakfast.
- Wait until at least 12:00 PM (noon) to take iron.
- Take iron with food if it upsets your stomach-this is fine, as long as itâs four hours after levothyroxine.
This works well for people who take their thyroid med right after waking up. But if iron makes you nauseous and you need to take it with lunch, youâre stuck waiting until after 4 PM to eat. Thatâs not always practical.
Strategy 2: Bedtime Levothyroxine, Morning Iron
- Take iron first thing in the morning, with breakfast or a snack.
- Wait until at least 3-4 hours after your last meal to take levothyroxine at bedtime.
This is the strategy many patients report as easiest to stick with. A 2024 survey by the Thyroid Patient Advocacy Group found that 58% of users who switched to nighttime levothyroxine said it improved their consistency. You donât have to rush in the morning. You donât have to skip lunch. You just take iron when you eat, and thyroid med before bed.
Important: Donât take levothyroxine right after dinner. Wait at least three hours. If you eat at 7 PM, take your thyroid med at 10 PM or later.
What About Other Iron Forms?
Not all iron supplements are the same. Ferrous sulfate is the cheapest and most common, but itâs also the strongest binder to levothyroxine. Ferrous gluconate and ferrous fumarate are slightly less interfering-but still problematic. Donât assume a âgentlerâ iron is safe to take together.Thereâs new hope: PharmacoLeverâs experimental chelated iron, called âThyroSafe Iron,â showed 87% less binding in early trials. But itâs not available yet. Until then, treat all iron supplements the same: separate by four hours.
What If Iron Makes You Sick?
Iron commonly causes nausea, constipation, or stomach cramps. Many patients take it with food to reduce this-but thatâs exactly when theyâre tempted to take levothyroxine too.Hereâs how to manage it:
- Take iron with vitamin C (like a glass of orange juice or a 250mg supplement). Vitamin C boosts iron absorption, so you might need a lower dose-reducing side effects.
- Try a slow-release iron pill. It releases iron slowly over time, which can mean less stomach upset. But it doesnât reduce the interaction risk.
- Split your iron dose: take half in the morning and half in the afternoon, both spaced four hours from levothyroxine.
Never skip iron because itâs hard to take. Iron deficiency worsens fatigue and can make hypothyroid symptoms feel worse. Fix the timing, not the supplement.
What You Should Monitor
If youâre just starting iron, or you changed your timing, get your TSH checked in 6-8 weeks. Thatâs how long it takes your body to stabilize.Watch for these signs your levothyroxine isnât working:
- Unexplained fatigue, even after sleeping
- Weight gain without changes in diet
- Feeling colder than usual
- Brain fog or memory issues
- Depressed mood
These arenât âjust aging.â Theyâre red flags your thyroid hormone isnât being absorbed.
Why Doctors Often Miss This
A 2024 JAMA Internal Medicine study found only 37% of primary care doctors consistently tell patients about this interaction. Many assume patients will read the label. Many assume the patient knows. But levothyroxine packaging varies. Generic brands often have vague instructions. Synthroidâs guide is clear. Others? Not so much.Doctors are busy. Patients are overwhelmed. This is why you need to take charge. Print out a simple schedule. Set phone alarms. Use the American Thyroid Associationâs free app. Donât wait for your doctor to remind you.
Real Stories, Real Results
On Redditâs r/Thyroid, one user wrote: âI took levothyroxine at 6 AM and iron at 10 AM with breakfast. My TSH jumped from 1.8 to 5.2. I switched to taking iron at 11:30 AM and levothyroxine at 10 PM. My TSH dropped back to 1.9 in six weeks. I finally feel like myself again.âAnother said: âI used to forget the timing. Now I set two alarms: 6:00 AM for thyroid, 12:00 PM for iron. I donât think about it anymore. It just happens.â
Itâs not about being perfect. Itâs about being consistent. Even missing the window once a week can push your TSH out of range over time.
Bottom Line: Do This Now
- If you take levothyroxine in the morning, wait at least four hours before taking iron.
- If you take iron in the morning, take levothyroxine at bedtime-three to four hours after your last meal.
- Never take them together. Not even if youâre in a hurry.
- Use vitamin C with iron to reduce side effects and dose.
- Check your TSH 6-8 weeks after changing your routine.
- Set alarms. Use a pill organizer with time labels. Write it down.
This interaction is preventable. Itâs not complicated. It just takes a little planning. Your thyroid depends on it.
jeremy carroll
December 15, 2025 AT 16:14bro i was taking iron at breakfast with my thyroid med for years and thought i was just getting older and slower... switched to bedtime levothyroxine and now i actually sleep through the night. no more 3pm crashes. life changed. đ
Edward Stevens
December 16, 2025 AT 12:52so let me get this straight... weâve got a 50-year-old medical fact that 84% of patients ignore, and the solution is... set two alarms? wow. groundbreaking. next youâll tell us to breathe air in between sips of coffee.
Alexis Wright
December 17, 2025 AT 19:17Letâs be honest - this isnât about timing. Itâs about the pharmaceutical-industrial complexâs deliberate obfuscation. Iron and levothyroxine binding? Thatâs textbook pharmacokinetic interference. But why do you think the FDA waited until 2022 to update labels? Because generics make billions selling unlabeled, untested combos. The real villain isnât the patient who forgot - itâs the system that lets them forget. And donât get me started on âThyroSafe Ironâ - patented in 2023, still not FDA-approved. Coincidence? Or corporate suppression?
People think this is about âcompliance.â Itâs not. Itâs about power. Who controls your absorption? Who profits when youâre tired, foggy, and back for another TSH test? The answer isnât in the guidelines - itâs in the stock ticker.
And yet, here we are, arguing over whether to take it at noon or 10 PM. Weâre being trained to solve symptoms, not systems. Wake up.
Rich Robertson
December 18, 2025 AT 14:26As someone whoâs lived with hypothyroidism for 18 years and iron deficiency since college, Iâve tried every combo. Morning thyroid + afternoon iron works if youâre a morning person. But if youâre like me - night owl, work late, eat dinner at 9 - bedtime thyroid is the only way. I take iron with my scrambled eggs at 7am, then my Synthroid at 11pm after my last sip of water. No alarms. No stress. Just routine. And yeah, my TSHâs been stable at 1.6 for two years now.
Also, vitamin C with iron? Non-negotiable. Orange juice or 250mg tab - makes the nausea vanish. And no, slow-release iron doesnât help with absorption interference. It just makes your poop less dramatic. Still separate by 4 hours.
Natalie Koeber
December 18, 2025 AT 15:27did you know the WHO quietly removed iron-thyroid interaction warnings from their 2021 global guidelines? they said it was 'overstated' because 'developing nations can't afford timing protocols.' but then the CDC re-added it in 2023... because big pharma paid them to. i've seen the emails. it's all a scam to sell more meds. your TSH isn't high - your system is being poisoned by corporate lies.
Rulich Pretorius
December 19, 2025 AT 18:48Let me offer a practical perspective from a country where access to healthcare is a lottery. In South Africa, many donât have the luxury of timing pills with military precision. But the principle remains: separation matters. If you can only manage two hours, do it. Then get your TSH checked. Itâs not about perfection - itâs about progress. Small steps, repeated, create stability. Donât let the perfect be the enemy of the functional.
And if youâre struggling with nausea? Try ferrous bisglycinate. Itâs pricier, yes, but gentler on the gut and still requires separation. But itâs not magic - just less brutal.
Dwayne hiers
December 20, 2025 AT 02:52Per the 2023 meta-analysis in Clinical Endocrinology (DOI:10.1111/cen.14892), the pharmacokinetic interaction between ferrous sulfate and levothyroxine is mediated by divalent cation chelation in the duodenum, reducing bioavailability via decreased passive diffusion across enterocytes. The four-hour separation window is empirically derived from Cmax and Tmax displacement curves - not arbitrary.
Furthermore, gastric pH variability in patients with H. pylori, PPI use, or atrophic gastritis further exacerbates the interaction. Therefore, the four-hour rule is conservative, not excessive. Patients with malabsorptive conditions (e.g., celiac, SIBO) may require six hours. Always titrate based on serial TSH and free T4, not anecdote.
Bedtime dosing is viable only if gastric emptying is delayed - which is common in hypothyroidism. But if youâre taking it with food, youâre compromising absorption regardless of timing. Fasting is non-negotiable for levothyroxine. Iron can be taken with food - thatâs the loophole.
Jonny Moran
December 20, 2025 AT 04:22Hey - I know this stuff is overwhelming. Youâre not dumb for forgetting. Youâre human. I used to take mine together too. Then I got a pill organizer with AM/PM slots and labeled them âTHYROIDâ and âIRONâ in big letters. Set a phone alarm for 10 PM. Thatâs it. No guilt. No panic. Just consistency. You got this.
Tim Bartik
December 21, 2025 AT 11:34lol u guys are so soft. in my day we just took our meds when we felt like it and if we felt like crap we blamed the government. iron is for weaklings anyway. if ur thyroidâs acting up, maybe u just need to lift more and stop drinking oat milk. america is falling apart because people canât even wait 4 hours for a pill. get a backbone.
Sinéad Griffin
December 22, 2025 AT 17:52OMG YES!!! I switched to bedtime thyroid and my skin stopped breaking out and my period came back?? I was so tired I thought I was going through menopause at 32 đ thank u for this post!! đđ #ThyroidWarrior #IronAndThyroid