GLP-1 Medications and Levothyroxine (Synthroid): The 33% AUC Increase the FDA Calls Out Only on Wegovy and Rybelsus, the Mechanism, and the TSH Monitoring Plan

About 1 in 8 American adults takes levothyroxine for hypothyroidism — roughly 30 million people [4]. If you are one of them and you start a GLP-1, the FDA has flagged a specific drug interaction worth knowing about: in the pivotal pharmacokinetic study (Hauge et al, Expert Opin Drug Metab Toxicol 2021, PMID 34289755) [1], total thyroxine AUC increased 33% (90% CI 1.25-1.42) after a single 600 mcg levothyroxine dose was co-administered with semaglutide tablets. The Wegovy Section 7.2 [5] and Rybelsus Sections 7.2 + 12.3 [6] prescribing information cite that finding verbatim and recommend “increased clinical or laboratory monitoring.” The Ozempic, Zepbound, Mounjaro, Saxenda, and Foundayo labels do not carry the same explicit warning [7, 8, 9] — but the underlying mechanism (delayed gastric emptying altering levothyroxine absorption) applies to every GLP-1 in principle [2]. The practical takeaway: re-check TSH 4-8 weeks after starting any GLP-1 and after every dose escalation, regardless of which one you take.

The pivotal study — Hauge et al, 2021

The single most-cited paper on this interaction is Hauge et al [1], an open-label, one-sequence crossover, single-center, multiple-dose, two-part trial in 45 healthy subjects published in Expert Opinion on Drug Metabolism & Toxicology in 2021. The headline findings:

• Total thyroxine AUC increased 33% (90% CI 1.25-1.42) after a single 600 mcg levothyroxine dose with concomitant semaglutide tablets.
• Cmax was unchanged — the peak hormone concentration was the same. Only the total exposure over time changed.
• Tmax was delayed — the peak occurred later because gastric emptying was slowed.
• The control arm with the absorption enhancer SNAC alone (without semaglutide) did not increase levothyroxine exposure, confirming the delayed-gastric- emptying GLP-1 component as the mechanism.

That clean mechanistic separation — SNAC alone is fine, SNAC + semaglutide produces the AUC bump — is what makes this study load-bearing for the clinical interpretation. The interaction is not about the oral semaglutide formulation per se; it is about GLP-1 receptor agonism slowing gastric emptying.

The verbatim FDA label language

Every Hauge finding is reflected in the Wegovy and Rybelsus labels — but interestingly, only those two. The differential matters for patient understanding.

Wegovy [5]

Wegovy Section 7.2 (Oral Medications) carries the verbatim statement:

In a drug interaction study with the semaglutide tablet, levothyroxine exposure was increased 33% (90% CI: 1.25-1.42).

Wegovy Section 7.2 also adds the broader recommendation: Consider increased clinical or laboratory monitoring for medications that have a narrow therapeutic index or that require clinical monitoring. Levothyroxine fits both criteria — the therapeutic-to-toxic window is narrow, and TSH monitoring is the standard clinical follow-up.

Rybelsus [6]

Rybelsus is the original source of the Hauge data because it is the oral semaglutide formulation that was actually studied. Section 7.2 and Section 12.3 (Pharmacokinetics) contain the most-detailed statement:

Levothyroxine: Total thyroxine (i.e., adjusted for endogenous levels) AUC was increased by 33% following administration of a single dose of levothyroxine 600 mcg concomitantly administered with semaglutide tablets. Maximum exposure (Cmax) was unchanged.

And the recommendation:

When using RYBELSUS or OZEMPIC tablets concomitantly with other oral drugs that have a narrow therapeutic index or that require clinical monitoring, consider increased clinical or laboratory monitoring.

Ozempic, Mounjaro, Zepbound, Saxenda, Foundayo — silence

The Ozempic [7], Mounjaro and Zepbound [8], Saxenda and Foundayo [9] labels do not contain a specific levothyroxine PK statement. Two reasons explain the silence:

• The Hauge et al PK study was conducted with semaglutide tablets (Rybelsus). Novo Nordisk did not run a separate study with semaglutide injection (Ozempic) or with the other GLP-1 agonists (tirzepatide, liraglutide, orforglipron), so the FDA does not have a formal trial result to cite on those labels.
• The mechanism is identical for any GLP-1 RA — receptor agonism slows gastric emptying — but the FDA label discipline requires a study-cited number, not a mechanism-extrapolated estimate. Hence the silence.

For patients, that distinction is more about regulatory process than clinical reality. A 2025 PBPK modeling study (Hooper et al, Pharmacotherapy 2025, PMID 39989027) [2] predicted that the GLP-1 class as a whole produces the same direction of effect on multiple oral drugs, including narrow-therapeutic-index medications. The clinical recommendation for a Mounjaro patient on levothyroxine should not differ meaningfully from that for a Wegovy patient — only the regulatory paperwork does.

The mechanism — gastric emptying and the levothyroxine absorption window

Levothyroxine absorption is unusually fragile. The drug is absorbed primarily in the jejunum (about 29% of dose) with smaller contributions from the duodenum (15%) and ileum, and absorption is heavily pH-dependent — tablet dissolution rate drops sharply when gastric pH rises above 3. That is why levothyroxine has the strict fasting rule (30-60 minutes before food, calcium, iron, or coffee) and why proton-pump inhibitors and antacids interfere with absorption.

GLP-1 receptor agonism slows gastric emptying — that is the same mechanism producing the early-satiety appetite suppression that makes the drugs work for weight loss. A delayed gastric emptying time means the levothyroxine tablet sits in the stomach longer, meeting more of the gastric acid before transiting to the absorption site, and arrives at the jejunum more fully dissolved. The result is the AUC bump documented in Hauge et al.

The Hooper et al PBPK study [2] modeled this same mechanism for several other oral drugs of clinical importance and predicted similar AUC effects: digoxin ~21% AUC increase, dabigatran 205% AUC increase. The size of the effect varies by drug (it depends on each drug's baseline absorption rate, narrow vs wide therapeutic index, and how quickly the gastric-emptying delay normalizes), but the directionality is consistent across the GLP-1 class.

What can actually go wrong — TSH suppression and iatrogenic hyperthyroidism

The most-cited published case demonstrating this interaction in real life is from the Journal of the American Pharmacists Association in 2024 (PMID 38992739) [3]. A post-total-thyroidectomy patient on stable levothyroxine for five years started subcutaneous semaglutide; TSH suppressed (signaling that the absorbed thyroxine dose was now too high), and the levothyroxine dose was reduced by 25% to restore euthyroidism.

Clinical iatrogenic hyperthyroidism from this mechanism produces the symptoms you would expect from too much thyroid hormone:

• Palpitations or feeling that your heart is racing
• Hand tremor or fine shakiness
• Heat intolerance or new sweating
• Insomnia or difficulty staying asleep
• Anxiety or new irritability
• Unintended weight loss beyond what the GLP-1 alone would explain
• New atrial fibrillation in older patients (the most serious downstream complication)

These symptoms can overlap with normal early GLP-1 side effects (the GI upset, the temporary fatigue), so they are not always obvious. A TSH check is the clean way to differentiate — a suppressed TSH on a patient who used to run TSH 1-2 mU/L is the signal.

The practical TSH monitoring schedule

Neither the American Thyroid Association nor the American Association of Clinical Endocrinologists has yet published a GLP-1-specific monitoring guideline. The practical schedule below borrows from the standard post-dose-change levothyroxine monitoring interval and extends it to the GLP-1 escalation timeline:

• Baseline TSH within the 3 months before starting the GLP-1, or have a recent TSH on file.
• Re-check TSH at 4-8 weeks after starting the GLP-1. The longer end of that range matches the steady-state pharmacokinetics of weekly semaglutide and tirzepatide.
• Re-check TSH 4-8 weeks after every dose escalation. The standard tirzepatide and semaglutide titrations add a dose increase every 4 weeks for several months; each escalation re-triggers the maximum gastric-emptying delay, so the AUC effect is not a one-time event.
• Re-check TSH any time you have unexplained palpitations, tremor, sleep disturbance, or weight loss beyond what the GLP-1 alone would predict.
• Once the GLP-1 dose is stable for 3+ months and TSH is stable, return to the patient's usual monitoring frequency (typically annually for stable hypothyroidism).

If TSH does drop into the suppressed range, the standard clinical move is a small (12-25%) levothyroxine dose reduction, not GLP-1 discontinuation. The hypothyroidism is still real; the levothyroxine dose simply needs to be re-titrated for the new absorption baseline.

Levothyroxine timing — does it help to space the doses?

A common patient-facing question is whether taking the morning levothyroxine pill 4 hours before or after the weekly GLP-1 injection mitigates the interaction. The published evidence does not support a clinically meaningful timing-based mitigation. Semaglutide and tirzepatide have half-lives of 5-7 days, so the gastric-emptying delay is essentially continuous between weekly injections, not pulsatile. Spacing the levothyroxine pill by a few hours from the injection does not change the chronic gastric-emptying state.

Keep the standard levothyroxine fasting routine (30-60 minutes before food, coffee, calcium, or iron). The TSH monitoring approach above does the work; timing tricks do not replace it.

Bottom line

• Roughly 30 million American adults take levothyroxine [4]; the GLP-1 + levothyroxine interaction is a real, regulator-recognized, well-mechanism-explained drug interaction that affects a meaningful fraction of GLP-1 patients.
• The Hauge et al 2021 PK study (PMID 34289755) [1] documented a 33% AUC increase in total thyroxine when levothyroxine 600 mcg was co-administered with semaglutide tablets; Cmax was unchanged.
• Wegovy Section 7.2 [5] and Rybelsus Sections 7.2 + 12.3 [6] carry the explicit warning verbatim. Ozempic, Mounjaro, Zepbound, Saxenda, and Foundayo do not [7, 8, 9] — but the mechanism applies to all GLP-1s and the practical monitoring recommendation is the same.
• Re-check TSH at 4-8 weeks after starting the GLP-1, after every dose escalation, and at any new symptoms of iatrogenic hyperthyroidism. If TSH is suppressed, expect a 12-25% levothyroxine dose reduction rather than GLP-1 discontinuation.
• Timing tricks (spacing levothyroxine away from the GLP-1 injection) do not replace TSH monitoring — the GLP-1 half-life makes the gastric-emptying effect continuous, not pulsatile.

Related research

• GLP-1 side effect questions answered — short-form Q&A on the most-searched GLP-1 safety topics, including hormone-related questions.
• Does GLP-1 cause cancer? MTC and thyroid evidence — a different thyroid concern (the medullary thyroid carcinoma boxed warning), worth understanding alongside the levothyroxine question.
• Mounjaro / Zepbound and birth control — companion piece on another oral-medication GLP-1-interaction story driven by the same gastric-emptying mechanism.
• Practical GLP-1 nausea management — covers the gastric-emptying physiology that drives both the desired appetite-suppression effect and the downstream oral-drug-absorption effects.
• How to take Foundayo (orforglipron) — relevant if you are weighing oral GLP-1 options alongside levothyroxine.