Most of us read a supplement label once, in the store, and then never again. The bottle goes into a cabinet, and the pills get taken whenever we remember, often all at once with breakfast.
For some combinations, that's fine. For others, the directions and warnings on the label are doing important work. To show how, here's a sample day for a woman in her late forties who takes thyroid medication, iron, calcium, a hair and nail supplement with biotin, and magnesium. Your own list will differ, but the questions are the same.
6:30 a.m.: thyroid medication, on its own
Levothyroxine is usually taken first thing, with water, on an empty stomach. What comes next matters. In a study of 20 people on a stable dose, taking 1,200 mg of calcium as calcium carbonate together with levothyroxine for three months lowered average free thyroid hormone levels and raised TSH from 1.6 to 2.7 mIU/L. One in five had a TSH above the normal range. Levels improved again after the calcium was stopped (Singh et al., 2000).
Iron can do something similar. When 14 patients took a 300 mg ferrous sulfate tablet at the same time as their thyroid medication for 12 weeks, average TSH rose from 1.6 to 5.4 mU/L, and 9 of them had more signs and symptoms of an underactive thyroid (Campbell et al., 1992).
Label habit: if you take any prescription medication, ask your pharmacist how far apart to take it from calcium, iron and other minerals. The right gap depends on the medication.
7:30 a.m.: breakfast and the "hair, skin and nails" capsule
The directions say "take with a meal," which is easy. The more important line is often missing from the front: the amount of biotin, listed in micrograms (mcg) on the Supplement Facts panel.
Biotin can interfere with lab tests. In a small study, 6 healthy adults took 10 mg (10,000 mcg) of biotin a day for a week, a dose the researchers described as common in over-the-counter supplements. Biotin interfered with 9 of 23 lab tests that use biotin in their design. Depending on the test, results came out falsely high or falsely low, and the tests studied included thyroid hormones, vitamin D and ferritin (Li et al., 2017).
Label habit: note the biotin amount, and tell whoever orders your blood work that you take it before the test is drawn.
10:30 a.m.: iron, away from calcium
If iron has been recommended, timing it away from calcium makes sense. In meal studies with 126 people, calcium reduced iron absorption in a dose-dependent way. Doses of 300 to 600 mg of calcium cut iron absorption by 50 to 60%, and so did 165 mg of calcium given as milk, cheese or a calcium salt (Hallberg et al., 1991).
So a mid-morning iron dose with water, rather than with a latte or a calcium tablet, keeps the two apart. If iron upsets your stomach, ask your doctor about options rather than simply stopping.
