Levothyroxine

At a glance

Also calledSynthroid, Levoxyl, Euthyrox, L-thyroxine, T4
L-thyroxine; T4; levothyroxine sodium. Brand names include Synthroid, Levoxyl, Euthyrox, Unithroid and Eltroxin. The synthetic form of the thyroid hormone thyroxine, taken to replace what an underactive thyroid does not make.
Typical dose1.6 mcg per kg a day, adjusted to thyroid blood tests
To replace thyroid hormone in an underactive thyroid: the full adult replacement dose is 1.6 micrograms (mcg) per kg of body weight a day, started lower in older people and in those with heart disease or at risk of an irregular heartbeat called atrial fibrillation, then adjusted by 12.5 to 25 mcg every 4 to 6 weeks (every 6 to 8 weeks in the at-risk group) until thyroid blood tests are normal; more than 200 mcg a day is seldom needed. Children start higher per kilogram, from 10 to 15 mcg/kg a day in newborns down to 2 to 3 mcg/kg in teenagers still growing, as set out on the US Synthroid label.

Myth check

Natural desiccated thyroid works better than synthetic levothyroxine, because it gives you both thyroid hormones, T3 as well as T4, and many people only feel like themselves again after they switch.

In blinded trials, desiccated thyroid has not beaten levothyroxine. In the trial most often quoted, 70 adults took each for 16 weeks without knowing which was which: symptoms and thinking tests came out the same, weight was about 3 lb lower on desiccated thyroid, and 34 preferred it, 13 preferred levothyroxine and 23 had no preference. A 2024 systematic review found that its two randomised trials showed no difference in quality of life or symptoms, that only weaker non-randomised studies favoured desiccated thyroid, and that long-term safety data are lacking; it notes desiccated thyroid is no longer recommended for an underactive thyroid. On the two hormones, T4 (thyroxine) and T3 (triiodothyronine), the levothyroxine label states that about 80% of the T3 in the blood is made from T4 in the body's own tissues.

Many people prefer desiccated thyroid and some feel better on it, but in blinded trials it has not worked better than levothyroxine.

The findingsStrongest first

OtherThe warning on the label is about soy flour, not isoflavones

Levothyroxine's labelling lists soybean flour alongside cottonseed meal, walnuts and dietary fibre as things that cut absorption, and warns against soy-based infant formula.

★☆☆Conflicting evidenceBalance hormones

2–

OtherIn 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.

★☆☆Conflicting evidenceBalance hormones

Sources2 behind this page

Evaluation of Levothyroxine Bioavailability after Oral Administration of a Fixed Combination of Soy Isoflavones in Post-menopausal Female VolunteersPersiani S et al., 2016DOI 10.1055/s-0035-1555784
SYNTHROID (levothyroxine sodium) tablet — current FDA structured product label, AbbVie Inc.AbbVie Inc., label version 1537, DailyMed published date 29 February 2024N/A — regulatory publication (DailyMed SPL setid 1e11ad30-1041-4520-10b0-8f9d30d30fcc, version 1537)

Taken with something else

What the evidence says about combining Levothyroxine with each of these. The marker is how well established the interaction is; a caution is about the hazard, which is a different question.

with Soy IsoflavonesRiskCautionLevothyroxine is already meant to be taken on an empty stomach, away from food, for reasons that have nothing to do with soy.★★☆Single study

What we don't know

2 open questions

Conflicting — The label names soybean flour and soy formula but gives no figure for how much absorption they cost and cites no study, so it cannot say how much a soy meal matters to the dose.
Conflicting — 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.