Levothyroxine replaces a hormone your body should be making, so at the right dose it should not cause side effects at all. Almost everything reported as a side effect is a sign the dose is too high or too low, which is a useful way to think about it: symptoms are information about the dose rather than a reaction to the medicine.
Absorption is easily disrupted by food, drinks and other medicines, so how and when you take it matters more than with most tablets.
Common side effects during long-term use
Signs the dose may be too high resemble an overactive thyroid:
- Palpitations or a racing heartbeat. See thyroid medicine and palpitations.
- Tremor, restlessness, anxiety or irritability. See thyroid medicine and anxiety.
- Difficulty sleeping
- Sweating, feeling too warm, or intolerance of heat
- Unintended weight loss, increased appetite, or loose stools
- Muscle weakness or cramps, and menstrual changes
Signs the dose may be too low are the symptoms of an underactive thyroid persisting: tiredness, feeling cold, constipation, weight gain, dry skin, low mood and slowed thinking. See signs your thyroid dose is too high.
Some hair thinning can occur in the first months of treatment and usually settles.
Serious or persistent symptoms to discuss with a clinician
Get urgent help for chest pain, a markedly fast or irregular heartbeat, or severe breathlessness.
Contact your doctor if over-replacement symptoms persist, because sustained excess thyroid hormone carries longer-term consequences: it can contribute to bone thinning and to atrial fibrillation, an irregular heart rhythm that becomes more likely with age. This is the main reason thyroid levels are checked rather than doses simply being left alone.
Older adults and people with heart disease are usually started on a lower dose and increased slowly for this reason.
Dose changes, missed doses and monitoring tests
Levothyroxine is usually taken on an empty stomach with water, well before food, and consistently at the same time each day. Consistency matters as much as timing: an absorbed dose that varies day to day makes blood results hard to interpret.
Blood levels take several weeks to reflect a dose change, so thyroid function is normally rechecked around six to eight weeks after any adjustment rather than sooner. Do not change your own dose based on how you feel in the interim.
Pregnancy increases thyroid hormone requirements, often early. Tell your doctor as soon as you know you are pregnant or are planning to be, so that levels can be checked promptly.
Different brands can differ slightly in absorption, so if your brand changes and you feel different, mention it.
Interactions with OTC medicines and supplements
Several very common products block levothyroxine absorption if taken at the same time. The usual solution is separating them by several hours rather than avoiding them.
- Calcium and iron supplements. Both bind levothyroxine in the gut. See calcium supplements and iron tablets.
- Antacids and indigestion remedies. Same effect. See antacid side effects.
- Stomach acid medicines. Long-term acid suppression can reduce absorption.
- Multivitamins containing calcium or iron, which is most of them. See multivitamin side effects.
- Coffee and soya products, which can reduce absorption if taken close to the dose.
Supplements marketed for thyroid support may contain iodine or actual thyroid hormone and can interfere with treatment. Check before taking any. See supplement and medicine interactions.
What to ask your doctor before changing treatment
- What was my last thyroid result, and when is the next check due?
- How long should I leave between my tablet and food, coffee or supplements?
- What should I do if I miss a dose?
- Could my symptoms mean my dose needs adjusting?
- Does anything I take over the counter affect absorption?
Related chronic medication guides
- Chronic medication side effects
- Managing chronic medication side effects
- Should I stop my chronic medication
- Over-the-counter medicines with chronic medication
- Report a side effect in South Africa
See also thyroid medication side effects and Eltroxin, a South African levothyroxine brand.
Medical Disclaimer: This content is for informational and educational purposes only and does not constitute medical diagnosis, prescription, or professional advice. Always read the patient information leaflet and consult a qualified doctor or pharmacist regarding any medicine or health concern.