In 12 women, isoflavones left levothyroxine absorption alone
Twelve postmenopausal women on stable levothyroxine took 60 mg of isoflavones with their tablet or six hours later. Peak and total exposure came out at 1.02 and 0.99 times.
What the study found
A 60 mg isoflavone dose taken with levothyroxine changed nothing
Twelve postmenopausal women on stable levothyroxine took the supplement either with their tablet or six hours later. Peak concentration and total exposure came out at 1.02 and 0.99 times the reference, well inside the 0.80 to 1.25 window used to call two exposures equivalent.
The caveat · Paid for by the supplement's maker
Twelve women, a single dose, no blinding, and the trial was funded and run by the company that makes the supplement tested. One dose cannot detect anything that builds up over weeks of taking the two together. Everyone enrolled was healthy and stably treated, not newly diagnosed.
What we don't know
A single isoflavone supplement dose left levothyroxine absorption untouched, yet the drug's label warns against soybean flour and soy formula. What separates the two is the form of soy, protein against isolated isoflavones.