If you take levothyroxine for an underactive thyroid, the main thing to know about supplements is not that any of them are dangerous, but that a few of them quietly stop your thyroid tablet being absorbed if you take them at the same time. The fix is timing, not avoidance. Get the timing right and you can take the supplements you need without blunting your medication.
The minerals that bind to levothyroxine
Four minerals are the usual culprits: calcium, iron, magnesium and zinc. Taken alongside levothyroxine, they bind to it in the gut and carry it out before it can be absorbed. Nothing harmful happens in the moment. The problem is slower and easier to miss: your thyroid level drifts down because less of each dose is getting in, and it can look as though you need a higher prescription when really the tablet is being intercepted at breakfast.
The four-hour rule
The standard fix, and the one in the prescribing guidance, is simple separation. Take levothyroxine first thing in the morning on an empty stomach. Take any mineral supplement, or a multivitamin that contains those minerals, at least four hours later. The same goes for high-fibre supplements like psyllium, which slow absorption in the same way. This one habit removes the whole problem, which is why it matters more than which supplements you take.
The biotin trap (a testing issue, not an absorption one)
Biotin, often sold for hair and nails, does not affect levothyroxine itself. But at the high doses common in beauty supplements, above around 5,000 micrograms, it can skew thyroid blood tests, making results read falsely high or low depending on the assay. That can lead to a dose change based on a wrong number. The fix is to stop high-dose biotin for a couple of days before any thyroid blood draw, and to tell whoever takes the blood that you have been on it.
The simple version
Take your thyroid tablet alone in the morning, push minerals and fibre to four hours later, and pause high-dose biotin before a blood test. Timing is the whole game here. If you want to check a specific supplement against your levothyroxine, the free checker will show you the call and the reasoning, including the separation rule where it applies.