The rest of this blog refers to adult humans. If a child, adolescent or animal is involved - SEEK HELP.
Of course, we should all be careful and always take the intended dose of levothyroxine. But mistakes happen:
- Taking too much because the tablet dosages have changed. Maybe having been used to taking two 50 microgram tablets, then switched to 100 microgram tablets, and carried on taking two tablets.
- Maybe the packaging has changed or it is a different make and so inadvertently took the wrong tablet(s), or the wrong number.
- Taking a dose a second time.
- Taking another levothyroxine tablet instead of taking something else.
There are numerous possibilities.
The first thing to say is to keep calm. It is very unlikely any significant harm will occur.
Context
When levothyroxine tablets are being trialled, it is standard to give a dose of 600 micrograms to perfectly healthy volunteers. That will obviously add to whatever their own thyroid glands have produced and released. We do not see any reports of volunteers having significant issues.
When people do a levothyroxine absorption test, it is standard to give them 1000 micrograms in one dose. These are people who need to take levothyroxine but appear not to absorb it fully. People seem to cope with this quite readily.
Reports of taking too much levothyroxine and needing treatment are extremely rare. That is, both reports as in statistics (number of events) and reports as written documents describing the cases.
A one-off extra dose
If someone takes an extra amount, similar to their usual daily dose or less, it is probably going to have very little effect.
Think of it as having taken their next dose a bit early.
Some who have done this have noticed precisely no impact.
If unsure, it would be reasonable to consider missing the next dose, or taking a part of it (For example, 100 micrograms rather than 150 micrograms.)
It would probably be sensible to skip the next dose. Possibly even the next few doses. But it is all too easy to overdo this and end up dropping from high thyroid hormone levels to low, very quickly.
Longer-term extra doses
When someone mistakes their prescription and does something like take a double-dose for weeks, they often feel effects. And this can be much more of an issue than a single extra dose.
However, in most cases they have only reached this point because they have tolerated it reasonable well until now. And something has made them wonder, re-check their dose, and realise the mistake.
Depending on circumstances it might be advisable to call NHS 111 or contact the GP.
The most difficult question is whether to revert to the original dose. Although it might seem obvious to do so, if someone has been fine on double that, their original dose might have been too low. And any changes mean that blood tests results will be difficult to interpret.
Timetable
The biggest danger from an excess of levothyroxine is when two factors combine.
First, initially there are no significant symptoms and the issue is dismissed as unimportant.
Second, the level of T3 rises to a maximum - mostly likely around 48 hours after ingestion. This could extend to four or five days.
In such a situation the person might have seen a doctor but left having been reassured. Only later do they start to get symptoms. And if T3 rises fast and high, that can be a true emergency. Further, the initial dismissal might mean that there had been no further treatment such as taking propranolol which can reduce the impact.
If someone has symptoms from taking too much, or the excess is large even without any current symptoms, contact NHS 111, 999, or whatever is available to you.
If someone has taken an unknown amount, especially a child or where the amount is suspected to be large, go straight for emergency help, 999 or whatever is available to you.
Do not wait for symptoms.
One fairly spectacular case was someone who took their dog's levothyroxine tablets - and ended up taking ten times their prescribed dose. Even in this case, they just reverted to their intended dose and all was well two months later.
Accidental Thyrotoxicosis Caused by Inadvertent Ingestion of Levothyroxine “Dog-Tabs” by a Veterinarian with Hypothyroidism
https://www.amjmed.com/article/S0002-9343(12)00118-0/fulltext
[i][b]helvella - Taking Too Much Levothyroxine[/b]
A discussion about the impact of taking doses of levothyroxine that are greater than have been prescribed.
Last updated 10/02/2025[/i]
Link to blog:
https://helvella.blogspot.com/p/helvella-taking-too-much-levothyroxine.html
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