With thyroid tablets, how you take them affects how much reaches your bloodstream, and that in turn affects both your blood results and how you feel. This is unusual: for most medicines the dose is the dose. Here, consistency in how you take it is part of the treatment.
If your results keep coming back unexpected, or you feel different without any dose change, absorption is often the explanation rather than the thyroid itself.
Common side effects during long-term use
At the right dose, thyroid tablets should not cause side effects. What people experience is nearly always over- or under-replacement:
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.
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, since sustained excess affects bone and heart rhythm over time.
Dose changes, missed doses and monitoring tests
The routine that gives the most consistent results: take it at the same time each day, on an empty stomach with water, and wait before eating or drinking anything other than water. Many people take it first thing and have breakfast later.
Consistency matters more than the specific time. If mornings are impossible, ask about an alternative routine, since a consistent evening approach is workable for some people. What causes problems is varying it day to day.
Brand changes are worth knowing about. Different manufacturers’ tablets can differ slightly in how much is absorbed. If your pharmacy switches you to a different brand and you start feeling different, that is a recognised phenomenon and worth mentioning rather than dismissing. Ask whether your levels should be rechecked.
Blood levels take weeks to reflect any change, so retesting is normally around six to eight weeks after an adjustment.
Interactions with OTC medicines and supplements
Things that block absorption if taken at the same time:
- Calcium and iron supplements, and multivitamins containing either
- Antacids and indigestion remedies
- Long-term stomach acid medicines
- Coffee, and soya products
- High-fibre supplements
The usual advice is separating these by several hours rather than avoiding them. Coffee catches people out most often, because taking a tablet with the first coffee of the day feels like taking it on an empty stomach.
Avoid supplements marketed for thyroid support, which may contain iodine or thyroid hormone.
What to ask your doctor before changing treatment
- How long should I wait after my tablet before food or coffee?
- My brand changed and I feel different. Should my levels be rechecked?
- Can I take it in the evening instead if mornings do not work?
- Which of my supplements need separating from it?
- What do I do if I miss a dose?
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 medication side effects.
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.