Vyvanse and Levothyroxine Interaction: Safety, Timing, and Clinical Guidance

Is there an interaction between Vyvanse and levothyroxine?

Two separate questions get mixed together here: whether one drug changes the other's absorption or metabolism, and whether their effects add up in the heart and blood vessels.

On absorption and metabolism, the Vyvanse label describes no problem with thyroid hormone. Lisdexamfetamine is an inactive prodrug; red blood cells convert it to dextroamphetamine and l-lysine, a route that does not involve cytochrome P450 enzymes [1]. The Vyvanse drug-interaction table covers MAO inhibitors, serotonergic drugs, CYP2D6 inhibitors, urinary alkalinizing agents, urinary acidifying agents and tricyclic antidepressants. Thyroid hormones are not in that table [1].

On the levothyroxine side, labeling names calcium carbonate, ferrous sulfate, bile acid sequestrants, ion exchange resins, antacids, sucralfate and proton pump inhibitors as drugs that can reduce absorption. The instructions differ by drug: calcium carbonate and ferrous sulfate are to be taken at least 4 hours apart from levothyroxine; bile acid sequestrants are to be taken at least 4 hours after levothyroxine, or TSH monitored; and for antacids, sucralfate and proton pump inhibitors the label calls for monitoring rather than a fixed spacing interval. Lisdexamfetamine is not among the drugs named.

On cardiovascular effect, both labels point the same way. Levothyroxine labeling lists sympathomimetics among drugs with additive cardiovascular effects with thyroid hormone. The Vyvanse label warns that CNS stimulants raise blood pressure by a mean of about 2 to 4 mm Hg and heart rate by a mean of about 3 to 6 bpm, that some patients have larger increases, and that all treated patients should be monitored for tachycardia and hypertension [1].

Do the doses need to be separated?

Neither label gives a spacing instruction between the two drugs.

Levothyroxine sodium is synthetic L-thyroxine, an oral prescription hormone [2]. Its tablet and capsule labeling says to take a single daily dose on an empty stomach, one-half to one hour before breakfast, and at least 4 hours before or after drugs known to interfere with absorption. The Vyvanse label says to take it orally in the morning with or without food and to avoid afternoon doses because of the potential for insomnia [1].

What each label says about the heart

The Vyvanse label says to avoid the drug in patients with known structural cardiac abnormalities, cardiomyopathy, serious cardiac arrhythmia, coronary artery disease or other serious cardiac disease, because sudden death has been reported with CNS stimulants at ADHD doses in that group [1].

Levothyroxine labeling approaches the same organ from the other side. It warns about cardiac adverse reactions in older patients and in people with heart disease, including atrial fibrillation risk, and says to start below the full replacement dose in those groups. Its list of effects from too high a dose reads much like a stimulant's: palpitations, tachycardia, arrhythmias, angina, heart failure, tremor, nervousness, insomnia, heat intolerance, sweating and weight loss.

Symptoms that appear on both lists

Insomnia, palpitations or fast heart rate, and weight loss appear in both labels' adverse-reaction lists. Tremor and sweating are listed only on the levothyroxine side. Levothyroxine labeling lists nervousness, while the Vyvanse label lists anxiety and feeling jittery [1].

In the 4-week adult ADHD trial, Vyvanse-treated patients reported insomnia 27% versus 8% on placebo, dry mouth 26% versus 3%, decreased appetite 27% versus 2%, anxiety 6% versus 0%, increased heart rate 2% versus 0% and palpitations 2% versus 0% [1]. Thyroid blood tests such as TSH can help assess whether the levothyroxine dose is appropriate, but a symptom such as a fast heart rate may have more than one cause.

Does Vyvanse change thyroid tests or the levothyroxine dose?

The Vyvanse label does not describe an effect on TSH or thyroid hormone levels, and its interaction table does not include thyroid products [1].

Is hyperthyroidism a contraindication to Vyvanse?

Not in the Vyvanse label. Its contraindications are known hypersensitivity to amphetamine products or other ingredients, and use of an MAO inhibitor, or within 14 days of stopping one, because of hypertensive crisis risk [1].

The related warning sits on the levothyroxine side, and it is about using thyroid hormone for weight loss. The levothyroxine boxed warning says thyroid hormones should not be used to treat obesity or for weight loss, that doses within the range of daily hormonal requirements are ineffective for weight reduction in euthyroid people, and that larger doses may produce serious or even life-threatening toxicity, particularly with sympathomimetic amines used for their anorectic effects. The Vyvanse label likewise states that it is not indicated for weight loss and that its safety and effectiveness for obesity have not been established [1].