Thyroid medication replaces a hormone your body should be producing itself. That makes it different from most medicines, and it explains the single most useful idea here: at the right dose it should cause no side effects at all.
Almost everything reported as a side effect is a sign the dose is too high or too low. Symptoms are information about the dose rather than a reaction to be tolerated, which means the answer is usually a blood test and an adjustment rather than accepting how you feel.
Common side effects during long-term use
Signs the dose may be too high, resembling an overactive thyroid: palpitations or a racing heartbeat, tremor, anxiety or irritability, difficulty sleeping, sweating and heat intolerance, unintended weight loss, loose stools, muscle weakness, and menstrual changes.
Signs the dose may be too low, meaning symptoms persist: tiredness, feeling cold, constipation, weight gain, dry skin, low mood and slowed thinking.
See signs your thyroid dose is too high, thyroid medicine and palpitations and thyroid medicine and anxiety.
Some hair thinning can occur in the first months of treatment and usually settles.
A frustrating situation worth naming: some people continue to feel tired despite blood results in the normal range. That is a real and recognised experience, and it is worth discussing rather than being dismissed, since other causes of tiredness may need looking at too.
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 symptoms of over-replacement persist. Sustained excess thyroid hormone contributes to bone thinning and to atrial fibrillation, an irregular heart rhythm more likely with age. That is the main reason levels are checked rather than doses being left indefinitely.
Older adults and people with heart disease are usually started on a lower dose and increased slowly for the same reason.
Dose changes, missed doses and monitoring tests
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 between.
Pregnancy increases thyroid hormone requirements, often early. Tell your doctor as soon as you know you are pregnant or are planning to be, so levels can be checked promptly. This is one of the more important points on this page and is easily missed.
Requirements also change with significant weight change, ageing, and some other conditions, so a dose that suited you for years may need revisiting.
Interactions with OTC medicines and supplements
Several very common products block absorption if taken at the same time. The usual fix is separating them by several hours rather than avoiding them.
- Calcium and iron supplements, including within multivitamins
- Antacids and indigestion remedies
- Stomach acid medicines taken long term
- Coffee and soya products 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.
What to ask your doctor before changing treatment
- What was my last result, and when is the next check due?
- Could my symptoms mean my dose needs adjusting?
- How long should I leave between my tablet and food, coffee or supplements?
- What should I do if I miss a dose?
- What should change if I become pregnant?
Related chronic medication guides
- Levothyroxine side effects
- Eltroxin side effects
- Chronic medication side effects
- Managing chronic medication side effects
- Report a side effect in South Africa
See also thyroid tablets and long-term thyroid medication.
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.