Levothyroxine replaces the hormone an underactive thyroid is not making. It is simple in principle and fussy in practice — how and when you take it changes how much gets absorbed, which is why the timing advice is not optional detail.
Take it once daily on an empty stomach, at least 30 minutes — ideally an hour — before breakfast or any other medicine, with water. Bedtime dosing works too, provided it is at least two hours after your last food.
Whichever you choose, be consistent. Moving between morning and evening changes absorption and muddies your blood test results.
Leave a four-hour gap between levothyroxine and:
Soya products, coffee and high-fibre foods also reduce absorption, which is another argument for the empty-stomach rule.
Your dose is set by blood tests, usually rechecked six to eight weeks after any change and then periodically. Do not adjust it yourself based on how you feel.
It takes several weeks for a dose change to show its full effect, so patience is part of the process.
Some people find they do better staying on the same manufacturer’s product. If you notice a difference after a switch, mention it — it can usually be accommodated.
Under-treated (dose too low): tiredness, feeling cold, weight gain, constipation, dry skin, low mood.
Over-treated (dose too high): palpitations, feeling hot and sweaty, tremor, anxiety, weight loss, diarrhoea, difficulty sleeping. Long-term over-treatment affects bone density and heart rhythm, so it is worth reporting rather than tolerating.
Contact your GP if you develop chest pain, a fast or irregular heartbeat, or significant unexplained weight change.
Levothyroxine is essential in pregnancy, and requirements usually increase. Tell your GP as soon as you know you are pregnant so your dose and monitoring can be adjusted — do not stop it.
If you miss a dose, take it as soon as you remember on the same day. If you only remember the next day, skip it and carry on — do not double up.
Do not stop levothyroxine because you feel well. Feeling well is the treatment working.
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