Selenium and Levothyroxine: Can You Take Them Together?

Levothyroxine comes with a routine of its own: one tablet with water as soon as you get up, then a wait before breakfast. Now a second bottle stands beside it, selenium at 200 mcg a capsule, bought after reading that the thyroid needs selenium. So can you take selenium with levothyroxine, and if so, where does it fit?

Usually yes: on the same day, not in the same swallow, and after a word with your doctor. Selenium is not among the supplements the MedlinePlus levothyroxine page says to keep away from the tablet, and we found no study showing that selenium lowers levothyroxine absorption. The simple routine is levothyroxine alone first thing and selenium later with a meal. The caution is the thyroid itself, where selenium has a job of its own. The documented timing problem involves calcium and iron, and both can hide in a multivitamin.

Do Selenium and Levothyroxine Interfere?

Not that we could find. According to MedlinePlus, levothyroxine "usually is taken once a day on an empty stomach, 30 minutes to 1 hour before breakfast." The page names two supplements to keep apart from it: "if you take calcium carbonate (Tums) or ferrous sulfate (iron supplement), take it at least 4 hours before or 4 hours after you take levothyroxine." Selenium does not appear on the page at all.

We also found no study showing that selenium reduces how much levothyroxine the body absorbs. That is an absence of evidence, not proof that no effect exists. It does mean there is no tested gap to follow, so the sensible routine is the one that protects the tablet's empty-stomach window.

The Real Timing Trap: Calcium and Iron

A calcium study shows what the 4-hour gap guards against. In a 2000 study in JAMA, Singh and colleagues had 20 patients on a stable levothyroxine dose take 1,200 mg of elemental calcium a day, as calcium carbonate, with their tablet for three months. During the calcium period, free T4 and total T4 fell and TSH rose, from 1.6 to 2.7 mIU/L; after the calcium stopped, the numbers recovered. It was a small study, and the calcium went down with the tablet itself, which is exactly the habit the gap is meant to prevent.

The practical difficulty is that calcium and iron are easy to take without noticing. Either can come in a multivitamin, and a blend sold for thyroid support may list minerals besides selenium. MedlinePlus names calcium carbonate and ferrous sulfate; if a combination product lists calcium or iron in any form, the cautious default is to give it the same 4 hours and ask your pharmacist if you are unsure. The "Tums" in the MedlinePlus wording is a reminder that calcium carbonate is also sold as an antacid, so a chewable taken for heartburn counts.

MedlinePlus also lists antacids and proton pump inhibitors among the nonprescription products that may interact with levothyroxine, and warns that foods and drinks containing soybeans, walnuts and dietary fiber "may affect how levothyroxine works for you." Our list of supplements you should not take together covers other pairs that need a gap.

Selenium, the Thyroid and Hashimoto's

People reach for selenium because of the thyroid, and there is a real link. According to the selenium fact sheet from the NIH Office of Dietary Supplements (ODS), selenoproteins, the proteins the body builds with selenium, "play critical roles in thyroid hormone metabolism, DNA synthesis, reproduction, and protection from oxidative damage and infection."

A role in thyroid hormone metabolism does not show that extra selenium helps a thyroid that is already being treated. In chronic autoimmune thyroiditis (Hashimoto's), trials of selenium that measured thyroid antibodies have given mixed results, according to ODS. The same fact sheet notes that "the American Thyroid Association recommends against selenium supplementation in pregnant women who have thyroid autoimmunity."

That is why selenium and thyroid medication get a caution label here. Selenium is not known to block the tablet, but it plays a part in the same hormone system your doctor is managing with it. Tell them before you start selenium, not at the next appointment.

Practical Recommendations

This is our suggested routine for timing supplements around levothyroxine, not a tested protocol.

• Levothyroxine first, and alone. Take it as prescribed, usually on an empty stomach before breakfast, and keep supplements out of that window.
• Selenium later, with a meal. Breakfast, after the wait, is the earliest slot. Lunch or dinner puts hours between the two if you prefer more distance. Pick one and keep it.
• Check every label for calcium and iron. Multivitamins and thyroid blends count. Anything that lists either goes at least 4 hours from the levothyroxine. If you take separate calcium and iron supplements, the ODS iron fact sheet notes that experts recommend taking those two at different times as well; our iron and calcium page covers that pair.
• Read the rest of the blend. If a thyroid formula also contains zinc, see our selenium and zinc page. If it contains ashwagandha, read our ashwagandha and thyroid guide before you take it.
• Leave the levothyroxine dose alone. Do not change it to make room for a supplement, or because a supplement seems to be working. Dose changes go through your doctor, who judges them by blood tests such as TSH.

Dosage Considerations

Selenium: the selenium upper limit for adults is 400 mcg a day, per the ODS fact sheet. Selenium can turn up in a single-mineral capsule, a multivitamin and a thyroid blend at once, so add up every label; our magnesium and selenium page works through an example. A 200 mcg capsule and a blend with 200 mcg already reach 400 mcg before the multivitamin is counted. Mind the unit, too: selenium is listed in micrograms (mcg), and a microgram is a thousandth of a milligram.

Calcium and iron: the Singh study tested 1,200 mg of elemental calcium a day. We found no study of smaller amounts, so the cautious default for calcium or iron in any product is the same 4-hour gap.

Who should check first:

• Anyone on levothyroxine who wants to add a supplement, selenium included. Ask your doctor or pharmacist how it fits around the tablet.
• Pregnant women with thyroid autoimmunity. The American Thyroid Association recommends against selenium supplementation for them.
• People with Hashimoto's hoping selenium will lower their antibodies. The trials are mixed, so make it a decision with your doctor rather than an experiment of your own.
• Anyone starting or stopping calcium or iron. In the Singh study, thyroid values moved when calcium was added and moved back when it stopped, so tell the doctor who manages your dose about either change.

Key Takeaway

We found no study showing that selenium interferes with levothyroxine, and MedlinePlus does not list it. Take the tablet alone first thing and selenium later with a meal, once your doctor agrees: selenium has a role in thyroid hormone metabolism, and its trial record in Hashimoto's is mixed. The documented timing trap is calcium and iron; keep them, multivitamins included, at least 4 hours from the tablet. Keep total selenium within 400 mcg a day, and leave the levothyroxine dose to your doctor.

Track Your Timing

Supplement Tracker sorts your supplements into time-of-day slots, each with its own reminder time, so selenium can sit at lunch while a calcium or iron product gets a slot at least 4 hours after your morning tablet. Check off each dose as you take it, and the dated history shows what you took and when, which is worth having if your doctor ever asks why a TSH result moved.