| Interaction type | Absorption. Iron forms an insoluble complex with levothyroxine in the stomach, so less thyroid hormone gets in. Iron itself is unaffected. |
| How serious | Moderate, and silent. You won't feel it happen; you'll feel the result weeks later as fatigue, cold, weight gain or hair shedding, all of which are easy to blame on menopause, PCOS, or the iron deficiency you were treating. |
| What to do | Levothyroxine first thing on an empty stomach, or at bedtime three to four hours after eating. Iron at least four hours away, usually with lunch or in the afternoon. Applies to every iron product: tablets, gentle "bisglycinate" forms, prenatals and multivitamins with iron. Recheck TSH six to eight weeks after starting or stopping iron. |
| Who this matters most for | Women on levothyroxine who are treating iron deficiency from heavy perimenopausal or PCOS periods; anyone taking a prenatal or multivitamin with iron; anyone whose TSH drifted up without an obvious reason. |
Medically reviewed by Dr Zainab Ajmal · Last reviewed September 2026 · Next review due September 2027 · MyStaq interaction record: levothyroxine-mineral-spacing--levothyroxine--iron
Why you are probably reading this
You take levothyroxine for an underactive thyroid. Heavy or unpredictable periods, common in both perimenopause and PCOS (now renamed PMOS), have dropped your iron, and someone has told you to take iron. Now you have two pills that both want the morning and a vague memory that they don't get along.
They don't. But this is one of the easiest interactions on the site to manage, and the management is timing.
What iron does to levothyroxine
Levothyroxine is absorbed in a narrow window in the small intestine, and it's fussy about company. Iron salts bind it in the stomach into a complex your gut can't take up; the FDA label describes a "ferric-thyroxine complex" [1]. The result is less thyroid hormone in your blood from the same tablet.
The defining study is from 1992 and still stands. Fourteen people on stable levothyroxine took 300 mg of ferrous sulfate at the same time as their thyroid tablet for 12 weeks. Average TSH rose from 1.6 to 5.4; it went up in 11 of the 14, and 9 of them developed symptoms of hypothyroidism [2]. A case report describes a pregnant woman who became hypothyroid when iron was added and hyperthyroid when it was stopped, with no change to her levothyroxine dose in between [3].
Two things follow:
- The effect is on levothyroxine only. Iron absorption isn't reduced by thyroid hormone. You're protecting the thyroid tablet, not the iron.
- It's about contact in the gut, not the day. Separate the two by enough hours that the levothyroxine is absorbed before the iron arrives, and the interaction disappears.
How far apart, and which iron
Four hours. The FDA label and the American Thyroid Association both say at least four hours between levothyroxine and iron-containing products [1], [4]. Some sources say two to four; four is the safe number and costs you nothing.
All iron counts. Ferrous sulfate, ferrous gluconate, ferrous fumarate, iron bisglycinate ("gentle iron"), polysaccharide iron, liquid iron, prenatal vitamins, and any multivitamin that lists iron on the panel. The interaction is chemical binding, and the form doesn't exempt it [1], [5]. Iron-fortified cereal at breakfast is a smaller version of the same problem, which is one reason levothyroxine belongs before breakfast, not with it.
Calcium is the same story. Calcium carbonate reduces levothyroxine absorption by the same kind of binding, with the same four-hour rule [1], [4]. If you take calcium for bone in postmenopause and iron for periods in perimenopause, both go in the afternoon.
Coffee, soy and fibre reduce absorption too, by different mechanisms. Levothyroxine wants water, an empty stomach, and 30 to 60 minutes alone [4].
Why this matters more if you have PCOS or you're in perimenopause
The symptoms hide inside each other. Under-replaced thyroid feels like fatigue, feeling cold, weight gain, low mood and hair shedding. So does iron deficiency. So does perimenopause. So, some days, does PCOS. If your TSH drifts because iron is blocking your levothyroxine, you will be tired for reasons that look like all three of the conditions you already have, and the pill timing will be the last thing anyone checks.
Iron deficiency itself weakens thyroid function. The enzyme that builds thyroid hormone, thyroid peroxidase, depends on iron, and iron deficiency reduces its activity [6]. So correcting a real iron deficiency helps your thyroid; you just have to correct it four hours away from your levothyroxine.
Hypothyroidism and PCOS travel together. Autoimmune thyroid disease is more common in women with PCOS than in women without it, so a woman with PCOS on levothyroxine treating heavy-period iron loss is not an edge case; she's a common one [7].
Trying to conceive or pregnant. Prenatal vitamins contain iron. Levothyroxine requirements rise early in pregnancy, often by a quarter to a half, and the thyroid association recommends checking TSH as soon as pregnancy is confirmed [4]. A prenatal taken with the levothyroxine can wipe out a dose increase your prescriber just made. Separate them from day one.
Only take iron if you're actually low. Iron fixes iron deficiency and does nothing helpful, and some harm, if you're not deficient. Get ferritin checked first. Heavy enough bleeding to need iron also deserves its own look; the iron is the follow-up, not the fix.
What to actually do
Pick a levothyroxine slot and defend it. Morning: on waking, with water, nothing else for 30 to 60 minutes. Or bedtime: at least three to four hours after your last meal, which studies show works as well or slightly better for some people [4]. Don't switch slots without telling your prescriber, because they'll want to recheck TSH after.
Put iron four or more hours away. Morning levothyroxine means iron with lunch or mid-afternoon. Bedtime levothyroxine means iron in the morning or with lunch.
Alternate-day iron is fine and may be better. Taking a single iron dose every other day increases the fraction absorbed compared with daily dosing, with less stomach upset [8]. It also makes the spacing easier.
Recheck TSH six to eight weeks after you start iron, stop iron, or change the timing [4]. If it has drifted up, the first question is timing, not dose.
Check every label. The iron that catches people is the one inside a "women's multivitamin" or a prenatal, not the one in the bottle marked iron.
What we do not recommend
- Iron and levothyroxine together "to keep it simple." The 1992 study is what simple looks like [2].
- "Gentle" or bisglycinate iron as a workaround. Different stomach, same chemistry [1], [5].
- Iron without a ferritin result. Iron for fatigue that isn't iron deficiency does nothing.
- Skipping the TSH recheck after starting iron because you feel fine. The 1992 group felt fine at first too [2].
- Taking levothyroxine with breakfast to fit iron in later. Food alone reduces absorption; move the iron, not the thyroid tablet [4].
How confident are we
| Claim | Evidence strength | Basis |
|---|---|---|
| Iron reduces levothyroxine absorption when taken together | Strong | Clinical study; FDA label; ATA guidance [1], [2], [4] |
| Four hours of separation prevents the interaction | Strong (guidance) | FDA label; ATA [1], [4] |
| The interaction applies to all oral iron forms | Moderate | Mechanism is chelation; guidance does not exempt any form [1], [5] |
| Iron deficiency impairs thyroid hormone production | Moderate | Enzyme studies and population data [6] |
| Alternate-day iron improves fractional absorption | Strong | Controlled absorption studies [8] |
| Bedtime levothyroxine is an acceptable alternative | Moderate | ATA; randomized crossover data [4] |
When to talk to a clinician rather than read further
- Your TSH has risen and you started iron, a prenatal or a multivitamin in the last few months
- You're pregnant or trying, and on levothyroxine
- You're taking iron without a ferritin or blood count result
- Periods are heavy enough to need iron (that needs its own assessment)
- You want to move levothyroxine to bedtime
None of this replaces your doctor. It is meant to make the conversation with them faster and better.
Quick questions
Can I take iron with levothyroxine? Not at the same time. Iron binds thyroid hormone in the stomach and blocks its absorption; in the defining study, TSH rose in 11 of 14 people who took them together. Take levothyroxine on an empty stomach and iron at least four hours later, and the problem disappears.
Does "gentle iron" or iron bisglycinate interact with levothyroxine too? Yes. The interaction is chemical binding, and every oral iron form does it, including the iron in prenatal vitamins and women's multivitamins. Keep them all four hours away.
How do I know if my iron is blocking my thyroid medication? You won't feel it directly. TSH creeps up over weeks, and you feel tired, cold or foggy, symptoms that look exactly like iron deficiency, perimenopause and PCOS. Recheck TSH six to eight weeks after starting iron, and if it has risen, fix the timing before changing the dose.
Sources
- Levothyroxine sodium tablets prescribing information (Synthroid and generics): Drug Interactions, drugs that may decrease absorption (ferrous sulfate, calcium carbonate; administer at least 4 hours apart).
- Campbell NR, Hasinoff BB, Stalts H, Rao B, Wong NC. Ferrous sulfate reduces thyroxine efficacy in patients with hypothyroidism. Ann Intern Med. 1992;117(12):1010-1013.
- Case reports summarised in: Simultaneous intake of liquid L-T4 formulation and iron salt: fact or fiction? 2023.
- Jonklaas J, Bianco AC, Bauer AJ, et al. Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement. Thyroid. 2014;24(12):1670-1751.
- National Institutes of Health, Office of Dietary Supplements. Iron Fact Sheet for Health Professionals (interactions with levothyroxine).
- Zimmermann MB, Köhrle J. The impact of iron and selenium deficiencies on iodine and thyroid metabolism: biochemistry and relevance to public health. Thyroid. 2002;12(10):867-878.
- Teede HJ, Tay CT, Laven J, et al. Recommendations from the 2023 International Evidence-based Guideline for the Assessment and Management of Polycystic Ovary Syndrome (screening for thyroid disease as a differential; thyroid disease prevalence). J Clin Endocrinol Metab. 2023;108(10):2447-2469.
- Stoffel NU, Cercamondi CI, Brittenham G, et al. Iron absorption from oral iron supplements given on consecutive versus alternate days and as single morning doses versus twice-daily split dosing in iron-depleted women: two open-label, randomised controlled trials. Lancet Haematol. 2017;4(11):e524-e533.
- MyStaq Health. Menopause guide (contraception, bleeding, iron and ferritin).
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These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease. Always consult your physician before starting any supplement, especially if pregnant, nursing, or taking medication.