Iron Supplements Compared: Ferrous Sulfate, Bisglycinate, Heme, Liposomal

Ferrous sulfate works but causes GI upset in roughly a third of users. Ferrous bisglycinate produces similar ferritin gains at lower doses with fewer side effects. Heme iron from red meat absorbs two to three times better than non-heme. Liposomal claims are not supported by absorption studies. Take iron every other day, and keep it at least four hours away from levothyroxine.

Why the form of iron matters

Iron supplements have to dissolve in stomach acid, get past competing minerals and polyphenols in the gut, and survive the body's own hepcidin "off switch" before they reach the bloodstream. Different formulations clear those hurdles differently, and that is what drives both efficacy and side effects [C1][C4].

Two principles do most of the work. First, heme iron (the form inside hemoglobin and myoglobin in animal tissue) is absorbed through a dedicated transporter and reaches the bloodstream two to three times more efficiently than the non-heme iron in plants, fortified cereals, and most supplements [C1]. Second, taking a single dose of iron raises hepcidin — the master regulator that blocks further absorption — for roughly 24 hours, which is why splitting doses or stacking them on consecutive days actually lowers net uptake [C2][C3][C4].

How the common forms compare

Ferrous sulfate. The cheapest and most studied form. Typical doses of 65 mg elemental iron per tablet are highly effective at restoring ferritin, but roughly a third of users report GI side effects — nausea, constipation, dark stools, abdominal pain — which is the leading cause of dropping the supplement [C4][C5].

Ferrous bisglycinate (and other amino-acid chelates). Iron bound to glycine. Two randomized trials in different populations have shown that lower doses of bisglycinate match or beat larger doses of ferrous sulfate. In Cambodian women, 18 mg of bisglycinate was non-inferior to 60 mg of ferrous sulfate over 12 weeks for ferritin gain — and the bisglycinate group had less gut inflammation [C5]. In a Danish pregnancy prophylaxis trial, 25 mg of bisglycinate matched 50 mg of ferrous sulfate at preventing iron deficiency, with fewer GI complaints [C6]. The take-home is that the chelated form is gentler at similar efficacy, which matters most for the patients who quit ferrous sulfate.

Heme iron (red meat, poultry, fish, "heme polypeptide" pills). Food-source heme iron is the most bioavailable form on a milligram-for-milligram basis [C1]. Heme polypeptide supplements claim the same advantage, but the supporting trials are small and the dose of elemental iron per capsule is usually low — so total intake is the limiting factor in practice [C1]. Whole-food heme iron, where tolerated, remains the most efficient way to deliver iron at meals.

Liposomal iron. The marketing pitch is that wrapping iron in lipid vesicles bypasses gut side effects and delivers more to the bloodstream. The strongest randomized data does not support that pitch: a 2022 crossover trial in healthy adults found liposomal iron produced a markedly smaller serum iron rise than ferrous sulfate, and a liposomal iron trial in dialysis patients underperformed intravenous iron — a low bar [C7]. The clinical data does not back the higher price tag.

Every-other-day dosing — the modern protocol

The Stoffel trials at ETH Zurich changed the standard advice. In iron-depleted but non-anemic women, total iron absorbed was significantly higher when 60 mg doses were given on alternate days versus consecutive days, and a single morning dose absorbed better than the same total split twice daily [C2]. A follow-up trial in iron-deficient anemic women confirmed the pattern: alternate-day dosing yielded greater fractional absorption per dose [C3].

The mechanism is hepcidin. One dose blunts absorption of the next dose for about 24 hours, so the "every-day, twice-a-day" regimens many of us were taught actually waste a meaningful share of each tablet. Skipping a day lets hepcidin fall and the gut re-open [C4]. The clinical translation: once-daily, alternate-day dosing is now reasonable for most non-pregnant iron-deficient adults; pregnancy and severe deficiency are situations where your endocrinologist or obstetrician may still recommend daily dosing [C4].

What blocks absorption — and what to do about it

Three interactions matter for hypothyroidism patients:

• Levothyroxine binds to iron salts in the gut and absorption drops sharply when they are co-ingested. The 2014 American Thyroid Association guidelines name iron supplements as one of the supplements that should be separated from levothyroxine; the standard rule is at least four hours apart [C8]. See our detailed calcium-iron-levothyroxine timing guide.
• Calcium (dairy, calcium supplements, antacids) cuts non-heme iron absorption in single-meal studies. Take iron away from your largest dairy meal and away from calcium supplements [C1].
• Tea and coffee polyphenols can reduce non-heme iron absorption by 50–90% when consumed with the dose [C1]. Wait at least an hour between iron and tea or coffee, and pair iron with vitamin C-rich foods (citrus, peppers, strawberries) to boost uptake.

What does NOT help

• "Slow-release" or "delayed-release" iron moves iron past the duodenum, the main absorption site, and consistently underperforms standard ferrous salts [C1][C4].
• Iron infusions for everyone with low ferritin. Reserved for true intolerance to oral iron, malabsorption, or severe symptomatic anemia — not a routine substitute for oral therapy [C4].
• "Iron + multivitamin" combo pills stack iron with calcium, zinc, and polyphenol-rich botanicals that compete for absorption. A standalone iron tablet on an empty stomach is more effective [C1].
• High-dose liposomal claims. The available absorption data do not support a meaningful advantage over standard ferrous salts [C7].

Practical guidelines

1. Start with ferrous sulfate or ferrous bisglycinate. Bisglycinate is the better-tolerated option if GI side effects are a concern; sulfate is cheaper and equally effective when tolerated [C5][C6].
2. Take iron once a day, every other day. Alternate-day dosing produces greater total absorption and fewer GI side effects than daily dosing for most non-pregnant adults [C2][C3][C4].
3. Take it on an empty stomach with vitamin C (a glass of orange juice or 100 mg ascorbic acid) to maximize non-heme absorption [C1].
4. Separate iron from levothyroxine by at least 4 hours, and from calcium-rich meals and tea/coffee by at least 1 hour [C1][C8].
5. Recheck ferritin at 8–12 weeks. Iron stores rebuild slowly; expect months, not weeks, to fully replete [C4].
6. Ask your endocrinologist before switching to liposomal or heme polypeptide products — they cost more without trial evidence of a clear advantage [C7].

Frequently asked questions

Is bisglycinate really gentler than ferrous sulfate? Yes, in the trials that compared them head-to-head, bisglycinate produced fewer GI complaints at lower doses while matching or beating sulfate on ferritin gain [C5][C6]. If you stopped a previous iron supplement because of nausea or constipation, bisglycinate is the reasonable next step.

Does liposomal iron actually work better? Available trials do not show a meaningful absorption advantage over standard ferrous salts, and one head-to-head study found liposomal iron underperformed sulfate on serum iron response [C7]. Marketing claims have outpaced the data.

Why every other day instead of every day? A single dose of iron raises hepcidin, which blocks the gut from absorbing more iron for about 24 hours. Skipping a day lets hepcidin fall and the next dose gets in [C2][C3][C4]. You absorb more total iron from three weekly doses than from seven.

Can I take iron with my levothyroxine to save time? No. Iron binds levothyroxine in the gut and your thyroid medication will work poorly. The American Thyroid Association recommends separating them; the standard rule is at least 4 hours [C8].

Will iron cure my hypothyroidism? No — iron does not cure hypothyroidism. But correcting documented iron deficiency can improve fatigue, hair, and the conversion of T4 to T3, all of which often overlap with hypothyroid symptoms. See our iron-deficiency-thyroid article.

Bottom line

For most adults with iron deficiency, once-daily ferrous sulfate or ferrous bisglycinate on alternate days, on an empty stomach, with vitamin C is the highest-yield protocol the current evidence supports [C2][C3][C4][C5][C6]. Bisglycinate is gentler for sensitive guts at similar efficacy [C5][C6]. Liposomal claims are not supported by absorption trials [C7]. Whole-food heme iron from meat is the most bioavailable dietary form [C1]. Whatever form you choose, separate it from levothyroxine by at least four hours [C8] — and your endocrinologist will retest ferritin in 8 to 12 weeks to confirm you are repleting.