Jardiance and Levothyroxine Interaction

No Direct Pharmacokinetic Interaction Exists

Empagliflozin and levothyroxine do not compete for the same metabolic pathways, and no published evidence supports a direct drug, drug interaction between them. The empagliflozin prescribing information confirms the drug undergoes glucuronidation via UGT2B7, UGT1A3, UGT1A8, and UGT1A9 with minimal CYP450 involvement, and it is neither a clinically significant inhibitor nor inducer of CYP enzymes or P-glycoprotein [1]. Levothyroxine, a synthetic form of endogenous T4, is not metabolized through CYP pathways either. It undergoes sequential deiodination in peripheral tissues (liver, kidney, and other organs) to produce the active hormone triiodothyronine (T3) [2].

This means the two drugs do not interfere with each other's breakdown or clearance. Major DDI databases (Lexicomp, Clinical Pharmacology, Micromedex) do not flag a rated interaction between empagliflozin and levothyroxine. The concern that does exist is entirely about gastrointestinal absorption, not systemic metabolism [3].

Levothyroxine Absorption Is Uniquely Fragile

Levothyroxine has a narrow therapeutic index and absorption characteristics that make it vulnerable to interference from nearly any co-administered oral substance. The Synthroid prescribing information states that "many drugs and foods affect levothyroxine absorption and may necessitate dosage adjustments" and recommends administration "as a single dose, preferably on an empty stomach, one-half to one hour before breakfast" [2]. Bioavailability on an empty stomach reaches approximately 80%, but drops to roughly 64% when taken with food, according to data reviewed in the American Thyroid Association (ATA) 2014 treatment guidelines [4].

The mechanism is straightforward. Levothyroxine is absorbed primarily in the jejunum and upper ileum. Substances present in the gut lumen (calcium, iron, proton pump inhibitors, bile acid sequestrants, and other oral medications) can bind to or otherwise reduce levothyroxine contact with the absorptive mucosa [3]. A 2009 review by Liwanpo and Hershman catalogued over 20 drug classes known to impair thyroxine absorption through adsorption, altered gastric pH, or chelation [3].

Empagliflozin does not fall into any of those high-risk categories. It is not a cation, does not alter gastric pH, and does not form insoluble complexes. Still, the general clinical principle applies: any tablet taken at the same time as levothyroxine introduces a theoretical absorption risk simply by competing for mucosal surface area and altering gastric transit.

SGLT2 Inhibitors and Thyroid Function Testing

Empagliflozin produces an osmotic diuresis that reduces plasma volume by approximately 7% in the first weeks of therapy, according to data from EMPA-REG OUTCOME (N=7,020), where hematocrit rose from a mean of 41.3% to 43.6% in the empagliflozin group [5]. This hemoconcentration effect can transiently alter the measured concentrations of protein-bound hormones, including total T4 and total T3.

Free T4 and TSH, which are the standard monitoring parameters for levothyroxine therapy, are less affected by volume shifts. The ATA 2014 guidelines recommend using TSH as the primary monitoring tool for dose adequacy, with free T4 as a secondary measure [4]. When a patient starts empagliflozin, clinicians should be aware that a modest rise in total T4 concentration does not necessarily indicate overreplacement. Checking free T4 and TSH (rather than total T4) avoids misinterpretation.

A practical monitoring framework for patients on both medications: measure TSH at baseline before adding empagliflozin, recheck at 6 to 8 weeks, and again at 6 months. If TSH shifts outside the reference range (typically 0.4 to 4.0 mIU/L), adjust the levothyroxine dose in 12.5 to 25 mcg increments rather than attributing the change to a "drug interaction."

Thyroid Disease Is Common in Type 2 Diabetes

The overlap between the patient populations taking these two medications is substantial. A meta-analysis published in the Journal of Diabetes Investigation found that the pooled prevalence of thyroid dysfunction among patients with type 2 diabetes was 11.0% (95% CI, 9.6% to 12.4%), compared with 6.6% in age-matched controls [6]. Hypothyroidism accounted for the majority of those cases. The Endocrine Society's 2012 clinical practice guideline on hypothyroidism notes that "unrecognized hypothyroidism may worsen dyslipidemia and glycemic control" in patients with diabetes [7].

This comorbidity pattern means that a large number of patients prescribed Jardiance for type 2 diabetes, heart failure with preserved ejection fraction (as studied in EMPEROR-Preserved, N=5,988 [8]), or chronic kidney disease will also be taking levothyroxine. The prescribing clinician should address dosing logistics at the point of co-prescription rather than assuming the patient will figure out timing independently.

Subclinical hypothyroidism (TSH 4.5 to 10 mIU/L with normal free T4) affects an additional 5% to 10% of the diabetic population and may go untreated. Starting an SGLT2 inhibitor in these patients does not worsen thyroid function, but the diuretic effect can shift TSH into a range that prompts a new levothyroxine prescription. Baseline thyroid screening before initiating empagliflozin is reasonable in patients with risk factors (female sex, age over 60, personal or family history of autoimmune disease).

Recommended Dosing Schedule When Taking Both Drugs

The simplest and most evidence-supported approach is a morning-first protocol for levothyroxine, followed by empagliflozin with or after breakfast.

Step 1. Take levothyroxine immediately on waking with a full glass of plain water. No coffee, food, or other medications for 30 to 60 minutes. The ATA guidelines specifically state: "levothyroxine should be taken either 60 minutes before breakfast or at bedtime (3 or more hours after the evening meal) for optimal absorption" [4].

Step 2. Take empagliflozin (10 mg or 25 mg) with breakfast or the first meal of the day. The empagliflozin FDA label permits administration with or without food and does not specify a time-of-day requirement [1].

Step 3. If a patient cannot tolerate morning levothyroxine (for example, due to nausea or GERD symptoms when lying flat), bedtime dosing of levothyroxine is an acceptable alternative. A 2010 randomized crossover trial (N=90) published in the Archives of Internal Medicine found that bedtime levothyroxine produced TSH levels 1.25 mIU/L lower on average than morning dosing, likely due to a longer fasting interval overnight [9]. In that scenario, empagliflozin is taken with breakfast and levothyroxine at least 3 hours after the last evening food or drink.

Patients who add calcium supplements, iron, or antacids should separate those by at least 4 hours from levothyroxine. This stacking of timing requirements is where medication reconciliation becomes important: the more co-medications a patient takes, the narrower the feasible dosing windows.

Monitoring TSH and Adjusting Levothyroxine

TSH measurement is the cornerstone of levothyroxine dose management. The 2014 ATA guidelines recommend targeting a TSH between 0.4 and 4.0 mIU/L for most adults, with a tighter target of 0.5 to 2.5 mIU/L sometimes preferred in younger patients or those planning pregnancy [4].

When empagliflozin is added to a stable levothyroxine regimen, the expected effect on TSH is zero if timing is maintained correctly. If TSH rises after starting empagliflozin, the differential includes:

• Absorption interference. The patient is taking both pills simultaneously.
• Hemoconcentration artifact. Check free T4 to confirm whether the TSH elevation reflects true underreplacement or a lab artifact.
• Intercurrent illness. Weight loss from SGLT2 therapy (mean 1.8 kg with empagliflozin 25 mg vs. placebo in EMPA-REG OUTCOME [5]) may reduce levothyroxine requirements in some patients, while caloric restriction or ketosis can transiently increase TSH.
• Non-adherence. The added complexity of timed dosing may reduce levothyroxine compliance.

A reasonable protocol: draw TSH and free T4 at 6 weeks, 3 months, and 6 months after co-prescribing. If stable at 6 months, return to the patient's routine monitoring interval (typically every 6 to 12 months). Document the dosing separation instruction in the after-visit summary and verify at each follow-up.

Empagliflozin's Broader Drug Interaction Profile

While levothyroxine is not a concern from a systemic interaction standpoint, empagliflozin does have clinically meaningful interactions with certain other drug classes. The FDA label identifies the following [1]:

Insulin and insulin secretagogues. Combining empagliflozin with insulin or sulfonylureas increases hypoglycemia risk. The label recommends considering a lower dose of insulin or the secretagogue when adding empagliflozin.

Diuretics. Loop and thiazide diuretics combined with empagliflozin can amplify volume depletion and orthostatic hypotension, particularly in patients over age 75. In EMPA-REG OUTCOME, the incidence of volume depletion events was 5.1% with empagliflozin vs. 3.2% with placebo among patients also taking loop diuretics [5].

Lithium. SGLT2-mediated osmotic diuresis can increase lithium reabsorption in the proximal tubule, raising serum lithium levels. Lithium monitoring is advised when starting or stopping empagliflozin [10].

These interactions are pharmacodynamic, not pharmacokinetic. Empagliflozin does not inhibit or induce CYP1A2, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4, P-gp, OAT3, OATP1B1, or OATP1B3 at therapeutic concentrations [1]. This clean pharmacokinetic profile is why the direct interaction with levothyroxine (or most other oral medications) remains theoretical rather than demonstrated.

Special Populations

Elderly patients (age 65 and older). Levothyroxine requirements often decrease with age because of reduced lean body mass and slower T4 clearance. The ATA guidelines note that "over-replacement with levothyroxine in the elderly is associated with atrial fibrillation and accelerated bone loss" [4]. Starting empagliflozin in an older patient on levothyroxine warrants closer TSH surveillance (every 6 weeks for 3 months) and conservative dose adjustments.

Patients with gastroparesis. Diabetic gastroparesis delays gastric emptying and unpredictably alters oral drug absorption. Both levothyroxine and empagliflozin absorption may vary from day to day. Liquid levothyroxine formulations (Tirosint-SOL) show more consistent absorption in patients with GI motility disorders and may be preferred when absorption is unreliable [2].

Post-thyroidectomy patients. These patients have zero endogenous T4 production and are entirely dependent on exogenous levothyroxine. Any absorption disruption has outsized clinical impact. The dosing separation protocol should be strictly enforced, and TSH monitoring at 4-week intervals (rather than 6 to 8 weeks) after adding empagliflozin is prudent.

Pregnant patients. Empagliflozin is not indicated during pregnancy. Levothyroxine requirements increase by 30% to 50% during the first trimester due to rising TBG levels and expanded plasma volume [4]. This combination scenario should not arise in clinical practice.