The UK's NICE NG145 thyroid guidance is often quoted as saying that, for levothyroxine, a dose of 1.6 micrograms per kilogram is recommended. This is a misinterpretation of the guidance.
What it actually says is:
1.3.6 Consider starting levothyroxine at a dosage of 1.6 micrograms per
kilogram of body weight per day (rounded to the nearest 25 micrograms)
for adults under 65 with primary hypothyroidism and no history of
cardiovascular disease.
https://www.nice.org.uk/guidance/NG145
It does NOT say that this is the right dose for anyone. It says to consider that as a starting dose. It is a suggestion. Which might then need to be adjusted. The guidance goes on about persistent symptoms, checking TSH, "optimal wellbeing", even looking at Free T4. But it gives effectively no guidance on how to manage changes to dosing. And does not even hint at a maximum dose.
It does not say this is a typical, likely or desirable eventual stable dose.
The guidance doesn't even appear to consider that 1.6 micrograms per kilogram could be too much - whether as a starting dose or long-term. Some who are clearly hypothyroid without treatment simply do not ever need this high a dose and achieve quite good results and acceptable laboratory results for TSH, Free T4 and Free T3 tests.
There is plenty of historical evidence that doses in the range 1.6 to 2.3 micrograms of levothyroxine per kilogram are expected. With the advent of oral solutions (liquid) and gelcaps, these numbers would need reassessment because they are usually expected to be absorbed somewhat better. And there is also plenty of evidence that some people simply do not absorb levothyroxine as effectively as many others.
Therefore, you cannot look on 1.6 micrograms per kilogram as anything other than an estimate based on historic approaches to treatment which is unlikely to seriously overdose in people who do really need levothyroxine.
And the issue about age 65? Well, again this is a starting dose. Not long term. Older people in particular are more likely to have absorption issues which can end up needing very much higher doses. (Younger people can also have these issues, but I'm making the point that there really is no ceiling for doses and high doses can be required to achieve sufficient absorption, regardless age.)
My own take on dosing by weight finds many reasons to question the approach in its entirety. But even NG145 does not say that this starting dose is a target or some sort of universal dosing.
helvella - Estimation of Levothyroxine Dosing in Adults
https://helvella.blogspot.com/p/helvella-estimation-of-levothyroxine.html
