Levothyroxine absorption and interactions: getting a steady dose

Levothyroxine is absorbed in the small intestine, and absorption varies with food and with other things in the gut. If one day it is taken with breakfast and the next on an empty stomach, the amount absorbed changes, and so does the blood hormone level. A fixed routine makes the dose predictable.

The NHS says to take it on an empty stomach 30 to 60 minutes before the first meal and to try to take it at the same time each day. Some people prefer a bedtime alternative; if you want to change timing, discuss it with your GP so the tests can be interpreted correctly.

The NHS says to avoid caffeinated drinks such as tea or coffee in the 30 to 60 minutes before your first meal. Many people are surprised that the morning cup counts, but the guidance is clear on caffeine within that window.

A practical pattern is levothyroxine with water on waking, then the hot drink and breakfast once the time has passed. If you take your tablet at the weekend later than on weekdays, the difference may be noticed on blood tests, so mention this.

The NHS lists calcium and iron supplements and antacids and other indigestion treatments among the things that interact. The NHS Gaviscon page names levothyroxine among the medicines not to take within 2 hours before or after, and the ferrous sulfate page says to leave a gap between iron and levothyroxine.

The NHS also names biotin because it can affect thyroid tests. This is a laboratory interference rather than an absorption effect, so a thyroid test result taken while on biotin may mislead. Tell whoever orders your test.

• Calcium and iron supplements.

• Antacids and indigestion treatments.

• Biotin (vitamin B7) before blood tests.

• St John's wort and other herbal products.

The NHS names carbamazepine, omeprazole, semaglutide, sertraline, beta blockers and digoxin. The way each interacts differs; some change how levothyroxine is processed, and others affect symptoms or monitoring, so the point is not to stop anything but to ensure the prescriber knows.

When a new medicine from this list starts or stops, an earlier thyroid test is often sensible. Ask the prescriber whether to recheck, because the need depends on the medicine and the person.

The NHS says you will have regular blood tests to check you are on the right dose, and that an incorrect dose can cause feeling or being sick, diarrhoea, headaches or problems sleeping. Treatment is usually lifelong for an underactive thyroid.

Do not change the dose to chase symptoms between tests. Tell the clinic about new symptoms, changes in weight, and any change in supplements or other medicines, so they can interpret the next result in context.

The NHS says levothyroxine can be taken during pregnancy if needed, and that more blood tests may be required because the dose is likely to change. Tell your GP or midwife as early as possible if you are pregnant or planning pregnancy.

Other changes, such as starting a new weight-loss medicine, beginning iron treatment or moving to a night shift, are all worth mentioning, since each can alter how much hormone you absorb.