Can a patient with hypothyroidism taking Synthroid (levothyroxine) 50mcg take Vyvanse (lisdexamfetamine)?

Can a Patient Taking Synthroid 50mcg Also Take Vyvanse?

Yes, a patient taking Synthroid (levothyroxine) 50mcg can take Vyvanse (lisdexamfetamine), but close monitoring of thyroid function and cardiovascular status is essential, as stimulant medications may unmask or exacerbate symptoms of both hypothyroidism and hyperthyroidism.

Key Considerations for Combined Therapy

No Direct Drug-Drug Interaction

• There is no documented pharmacokinetic interaction between levothyroxine and lisdexamfetamine, as they are metabolized through different pathways and do not share common metabolizing enzymes 1.
• Levothyroxine is primarily metabolized through deiodination and conjugation, while lisdexamfetamine is converted to dextroamphetamine through hydrolysis 1.

Critical Monitoring Requirements

Thyroid Function Monitoring:

• Monitor TSH and free T4 every 6-8 weeks initially when starting Vyvanse, as stimulant medications can affect metabolic rate and may alter levothyroxine requirements 2, 3.
• Target TSH should remain within the reference range of 0.5-4.5 mIU/L to avoid complications from either under- or overtreatment 2, 3.
• Once stable on both medications, continue TSH monitoring every 6-12 months or if symptoms change 2.

Cardiovascular Monitoring:

• Obtain a baseline ECG before starting Vyvanse, particularly if the patient is over 60 years or has any cardiac risk factors, as both medications can affect heart rate and rhythm 2.
• Monitor for tachycardia, palpitations, or arrhythmias at each follow-up, as stimulants increase cardiovascular workload and levothyroxine overtreatment can cause atrial fibrillation 2, 3.

Symptoms That May Overlap or Confound Diagnosis

Hypothyroidism symptoms that may be masked by Vyvanse:

• Fatigue (68-83% of hypothyroid patients) may improve with stimulant therapy, potentially masking inadequate thyroid replacement 3.
• Weight changes become difficult to interpret, as hypothyroidism causes weight gain (24-59% of patients) while Vyvanse typically causes weight loss 3.
• Cognitive issues (45-48% in hypothyroidism) such as difficulty concentrating may improve with Vyvanse, obscuring the need for levothyroxine dose adjustment 3.

Hyperthyroidism symptoms that may be amplified by Vyvanse:

• Tachycardia, tremor, and anxiety can occur with both levothyroxine overtreatment and stimulant use, making it difficult to determine the cause 4.
• If TSH becomes suppressed (<0.1 mIU/L), reduce levothyroxine dose by 25-50 mcg immediately, as prolonged suppression increases risk for atrial fibrillation 3-5 fold, especially in patients over 60 2.

Specific Dosing Considerations

Levothyroxine Dose Adequacy

• A 50mcg dose is relatively low and may represent either early treatment, elderly patient dosing, or residual thyroid function 2, 4.
• Verify that TSH is within target range (0.5-4.5 mIU/L) before starting Vyvanse, as inadequate thyroid replacement can worsen fatigue and cognitive symptoms that Vyvanse is meant to treat 2, 3.
• If TSH is elevated (>4.5 mIU/L), increase levothyroxine by 12.5-25 mcg before attributing symptoms to ADHD 2.

Timing of Medication Administration

• Levothyroxine must be taken on an empty stomach, 30-60 minutes before breakfast, for optimal absorption 2, 5.
• Vyvanse can be taken with or without food but should be taken in the morning to avoid insomnia 3.
• Maintain at least 4 hours separation between levothyroxine and any medications that may interfere with absorption, though this does not apply to Vyvanse 2.

Common Pitfalls to Avoid

Do not attribute all symptoms to ADHD without confirming adequate thyroid replacement:

• Approximately 25% of patients on levothyroxine are unintentionally maintained on inadequate doses, leading to persistent hypothyroid symptoms that may mimic ADHD 2.
• Cognitive issues, fatigue, and difficulty concentrating occur in 45-48% of hypothyroid patients and will not improve with stimulants alone if thyroid function is inadequate 3.

Do not overlook cardiovascular risks in elderly patients:

• For patients over 70 years or with cardiac disease, both medications require more conservative dosing and closer monitoring 2, 4.
• The combination increases risk for unmasking cardiac ischemia or precipitating arrhythmias, particularly if levothyroxine dose is increased too rapidly 2.

Do not adjust levothyroxine dose based on symptoms alone while on Vyvanse:

• Wait 6-8 weeks after starting Vyvanse before making any levothyroxine dose adjustments, as stimulant effects on energy and cognition may temporarily mask hypothyroid symptoms 2, 5.
• Always confirm with TSH and free T4 testing rather than relying on clinical assessment alone 2, 3.

Special Populations Requiring Modified Approach

Patients with cardiac disease:

• Start Vyvanse at the lowest possible dose and titrate slowly, as the combination with levothyroxine increases cardiac workload 2, 4.
• Consider more frequent monitoring (every 2 weeks initially) rather than waiting 6-8 weeks 2.

Elderly patients:

• Use lower starting doses of both medications and titrate more gradually to avoid cardiac complications 2, 4.
• Target TSH may be slightly higher (up to 5-6 mIU/L) in very elderly patients to avoid overtreatment risks, though this remains controversial 2.