Medicine reading guide
Levothyroxine
Oral levothyroxine sodium tablets for thyroid-hormone replacement; other formulations and cancer suppression need specific instructions.
How it works
Levothyroxine supplies T4, the hormone a healthy thyroid normally makes. Tissues convert some T4 into the more active T3. Thyroid hormones act inside cells to regulate gene activity and protein production, helping control metabolism, growth, and normal body function. The medicine replaces a missing signal; it does not directly repair the thyroid gland.
Original content record dated 2026-09-30. Educational draft; independent clinical review pending.
About the referenced label
Selected label version 7 · SPL effective date 2026-05-20 · Retrieved 2026-10-09.
Label record supplied by Preferred Pharmaceuticals, Inc.. This may be a repackager's record.
These dates describe the source record, not a clinical review of this explanation. The online label may change; this explanation has not been revalidated against the retained snapshot by this source check.
Uses / Symptoms
- Underactive thyroid
- Selected thyroid cancers: specialist use
Effects
- Replaces missing thyroid hormone
- May improve tiredness and cold sensitivity
- Not a weight-loss medicine
Side effects
- Excess replacement: palpitations or tremor
- Sweating, insomnia or diarrhea
- Overtreatment can harm heart rhythm and bones
Selected uses and effects; not a complete safety list. Consult the referenced product information.
Explore the Levothyroxine mechanism GIF and named chapters →
Questions about Levothyroxine
What is this medicine used for?
Levothyroxine replaces thyroid hormone when the body makes too little. It is used for primary, pituitary, or hypothalamic causes of hypothyroidism. Specialist regimens also suppress TSH in selected thyroid cancers. It is not a weight-loss medicine for someone with normal thyroid function, and the treatment target differs between replacement and cancer suppression.
Why might it be prescribed for me?
Levothyroxine is chosen when thyroid hormone needs replacing, with the target matched to primary or central hypothyroidism or specialist cancer suppression. The starting approach changes with age, cardiac disease, pregnancy, and disease severity. Fatigue alone without a supported thyroid diagnosis is not enough to establish that thyroid hormone will help.
What improvement should I expect?
Replacing missing thyroid hormone can improve symptoms such as tiredness, cold sensitivity, constipation, and slowed thinking. The aim is an appropriate hormone level and better function, not stimulation or a rapid energy boost. Some symptoms have other causes, so persistent tiredness despite appropriate thyroid tests should be assessed rather than automatically increasing the dose.
How soon should it start working?
Symptoms may begin improving over several weeks, and the peak therapeutic effect of a dose may take 4–6 weeks. Laboratory review commonly follows at 6–8 weeks after an adult dose change. It is not a rapid stimulant. Persistent symptoms call for a review of thyroid tests, adherence, absorption, and other causes rather than daily dose changes.
How will I know whether it is helping?
For primary hypothyroidism, TSH is generally checked 6–8 weeks after a dose change and periodically once stable. Central hypothyroidism requires free T4-based assessment because TSH can mislead. Pregnancy and children need more frequent review. Track symptoms too, but do not adjust solely on energy, weight, or a single result without context.
How does it work, in simple terms?
Levothyroxine supplies T4, the hormone a healthy thyroid normally makes. Tissues convert some T4 into the more active T3. Thyroid hormones act inside cells to regulate gene activity and protein production, helping control metabolism, growth, and normal body function. The medicine replaces a missing signal; it does not directly repair the thyroid gland.
How much is usually taken, and how often?
The daily dose is individualized using body size, age, heart health, pregnancy, diagnosis, and thyroid blood tests. Doses are measured in micrograms, not milligrams. Do not copy another person's dose or increase it for energy or weight loss. Older adults and people with cardiac disease often start lower and need slower adjustment.
Should I take it with food?
Take the tablet once daily on an empty stomach, 30–60 minutes before breakfast, with water. Separate medicines that reduce absorption, such as calcium or iron, by at least four hours. Keep the routine consistent. If a different schedule is necessary, agree it with the clinician so blood tests can be interpreted against a stable routine.
What should I do if I miss a dose?
Take a missed dose when remembered unless it is nearly time for the next dose; then skip it. Do not double up unless your clinician has given an explicit different plan. Record missed or extra doses, since the pattern matters when interpreting thyroid tests. For an accidental extra dose, ask a pharmacist or Poison Control rather than guessing a correction.
What if I accidentally take too much?
Call US Poison Control at 1-800-222-1222 after an overdose or significant dosing error, even if you initially feel well: symptoms can be delayed. Have the strength in micrograms, amount, and timing available. Call 911 for chest pain with severe symptoms, collapse, seizure, serious breathing difficulty, or inability to wake.
How long might treatment continue?
Replacement is often long-term and may be lifelong when thyroid function cannot recover, such as after thyroid removal. Some causes of low thyroid hormone can change over time, so the prescriber determines duration. A dose may take several weeks to show its full effect; feeling better generally means replacement is working, not that it is no longer needed.
Can I stop when I feel better?
Do not stop levothyroxine just because you feel better. It replaces hormone and does not usually cure the reason it was low. A planned withdrawal or dose reduction needs a clinician-led reason and follow-up thyroid tests. There is no universal taper suitable for every diagnosis, especially pregnancy or thyroid-cancer suppression.
What side effects are common?
Too much replacement can cause palpitations, tremor, sweating, heat intolerance, insomnia, diarrhea, or weight loss. Persistent overtreatment can affect heart rhythm and bone health. Report these symptoms for dose and test review. Symptoms alone cannot determine the correct dose, and simply lowering it without assessment can leave hypothyroidism undertreated.
Which symptoms need urgent help?
Seek urgent medical help for chest pain, a fast or irregular heartbeat with feeling unwell, marked shortness of breath, or a severe allergic reaction. Call 911 for collapse or serious breathing difficulty. After a substantial extra dose, contact Poison Control even without symptoms because thyroid-hormone toxicity can appear later.
Can it interact with medicines or supplements?
Calcium, iron, and some binding medicines reduce absorption and need separation. Acid-suppressing medicines, food patterns, and other drugs can change the required dose. Warfarin and diabetes treatments may need closer monitoring when thyroid treatment changes. Tell the team about biotin supplements because they can distort some thyroid tests, even without changing thyroid function.
What about alcohol, driving, and exercise?
Keep food and supplement timing consistent so absorption is predictable. Levothyroxine is not intended to stimulate exercise performance or cause weight loss. If palpitations, dizziness, or breathlessness occur, seek assessment before strenuous activity or driving while impaired. This review does not establish a special alcohol restriction for levothyroxine alone; other conditions and medicines may do so.
What about pregnancy, breastfeeding, or fertility?
Adequate thyroid hormone is important during pregnancy, and levothyroxine should not be stopped because pregnancy occurs. Dose needs can rise; notify the clinician promptly for testing and adjustment. Follow-up is more frequent during pregnancy and after delivery. Replacement is compatible with breastfeeding when appropriately prescribed, with the dose guided by the mother's clinical and biochemical needs.
Will I need tests or monitoring?
For primary hypothyroidism, TSH is generally checked 6–8 weeks after a dose change and periodically once stable. Central hypothyroidism requires free T4-based assessment because TSH can mislead. Pregnancy and children need more frequent review. Track symptoms too, but do not adjust solely on energy, weight, or a single result without context.
Is there a generic or a lower-cost option?
Levothyroxine is available as a generic and in brands such as Synthroid, Levoxyl, and Unithroid; other formulations have their own names and instructions. Keep a record of the preparation and microgram strength. Prices and stock were not checked. Ask the pharmacy about an equivalent option and tell the clinician about product changes so follow-up can be planned.
What if it is not helping?
If symptoms or tests remain abnormal, first review missed doses, administration timing, interacting medicines, absorption, and the diagnosis. A formulation change may help selected patients but should be monitored. Adding T3 or using thyroid extracts is not a routine substitute established by this label; that decision needs a separate specialist evidence discussion.