Levothyroxine: uses, timing and interactions

Levothyroxine is a synthetic version of thyroxine, the main hormone made by the thyroid gland. The emc summary of product characteristics says its chief action is to increase the rate of cell metabolism, and that the body converts it to the active form, triiodothyronine, in tissues. The NHS says it treats an underactive thyroid, whose features it lists as tiredness, weight gain and low mood.

Emc also lists congenital hypothyroidism in infants and acquired hypothyroidism in children among licensed uses. Brands on emc include Eltroxin, alongside several generic tablets.

Take one dose a day, swallowed whole with water, on an empty stomach, at roughly the same time every day. The NHS says to leave thirty to sixty minutes before breakfast, caffeinated drinks or other medicines. A liquid form exists, measured with the syringe or spoon supplied. The NHS notes that most people need treatment for the rest of their life, so it becomes a habit rather than a course.

The emc information describes starting low and raising the dose in steps every few weeks in adults, with a gentler start for older people and for anyone with heart disease. Because those figures depend on age, weight and heart health, follow your prescriber and the leaflet.

Dose is guided by thyroid blood tests, particularly TSH, so you will have regular checks, especially after any dose change. The NHS says the wrong dose can cause nausea, vomiting, diarrhoea, headaches or sleep problems. Prolonged over-treatment can lead to bone loss and heart rhythm problems according to the emc.

The emc also warns that switching between brands may cause symptoms even when thyroid tests are normal. If you notice a change after a different make is dispensed, mention it to your pharmacist or prescriber instead of stopping or adjusting the tablets yourself.

The NHS lists insomnia, headaches, anxiety, sweating and hair loss as common effects, which often point to a dose that is not yet right. Rarely, anaphylaxis (swollen throat or tongue, rash, breathing difficulty) or thyroid crisis (high temperature, irregular heartbeat, dizziness, jaundice) can occur. For emergencies, call 999; for other concerns, speak to your pharmacist, prescriber or NHS 111.

• Call 999: swelling of the throat or tongue, breathing difficulty.

• Urgent advice: high temperature with an irregular heartbeat, or yellowing of the skin.

The NHS names carbamazepine, omeprazole, semaglutide, sertraline, antacids and indigestion remedies, calcium and iron supplements, beta blockers, digoxin, St John's wort and biotin (vitamin B7). The emc says antacids, calcium, iron supplements and proton pump inhibitors reduce absorption and should be separated from levothyroxine by about four to five hours. It also says levothyroxine can strengthen warfarin and may alter diabetes medicine needs, and orlistat can reduce absorption.

Biotin matters because it can distort thyroid blood test results; tell the person ordering your tests if you take it.

The NHS says levothyroxine may not be suitable if you have had a previous allergic reaction or serious side effect, have an overactive thyroid, diabetes, adrenal gland conditions or heart conditions, so tell your prescriber about these. The emc recommends caution in cardiovascular disease, adrenal insufficiency and diabetes.

The NHS says levothyroxine can be taken in pregnancy when needed, with extra blood tests and a discussion of risks and benefits, and the emc points out that untreated hypothyroidism carries greater risk to the baby. Breastfeeding is usually fine, with low levels in milk. Driving is not usually affected. If you forget a dose, take it when you remember unless the next dose is nearly due; never double up.

It is prescription only. Suspected side effects can be reported through the Yellow Card scheme.