Levothyroxine
Standard treatment for an underactive thyroid.
VIDEO LESSON
How it's taken
• Once daily on an empty stomach, 30 to 60 minutes before breakfast, or at bedtime well after the last meal
• Separate from calcium, iron and some antacids by about 4 hours
Side effects at a glance
At the right dose, side effects are uncommon. Too much causes palpitations, anxiety, insomnia and, over time, bone loss and atrial fibrillation.
Monitoring
TSH 6 to 8 weeks after any dose or brand change, then yearly once stable.
Good to know
Pregnancy usually needs a dose increase right away. Switching brands or generics can shift levels, so recheck TSH.
AT A GLANCE
Class
Thyroid
Brand names
Synthroid, Levoxyl, Unithroid, Euthyrox, Tirosint
Status
FDA-approved
Reviewed by
Farhan Tahir, MD
Is this medicine right for you?
Prescription medicines need individual medical advice. Dr. Tahir’s team offers consults through Youthful Medicine.
MECHANISM
How it works
Synthetic T4, identical to the main hormone the thyroid makes. Tissues convert it into active T3, which sets metabolic rate, heart rhythm and energy use.
THE EVIDENCE
What the research actually shows.
EVIDENCE SUMMARY
Levothyroxine is the standard treatment for overt hypothyroidism (high TSH with low free T4). For mild, subclinical hypothyroidism, the TRUST trial in adults aged 65 and older found no improvement in symptoms or tiredness compared with placebo, so treatment is individualized rather than automatic.
KEY TRIALS
TRUST
TYPICAL RESULT
Normalizes TSH in overt hypothyroidism
SAFETY
Risks, side effects and who should avoid it
Over-replacement can trigger atrial fibrillation and bone loss, especially in older adults. Take it on an empty stomach and apart from calcium, iron and other medicines that block absorption. TSH is rechecked 6 to 8 weeks after any dose change.
This summary is not a complete list of risks. Read the official prescribing information and discuss your own history with your clinician.
KEY REFERENCE
Stott DJ et al. Thyroid hormone therapy for older adults with subclinical hypothyroidism. N Engl J Med 2017;376:2534–2544.
Educational content only, reviewed by Farhan Tahir, MD. Not individual medical advice. Medicines named here require a prescription and medical supervision.