How much levothyroxine do we get from a single tablet?
Taking just two factors into account it is possible to see a dramatic variation in the amount of levothyroxine we actually get from a single tablet.
Tablet Potency
In the UK, levothyroxine has to be within the range of 90 to 105% potent. (In the USA, the permitted range is 95 to 105%.) That is, when tested, a 100 microgram tablet must have between 90 and 105 micrograms of levothyroxine. This potency must be achieved from the moment the tablets are manufactured right through to expiry date.
In the real world, tablets can be expected to be in a much narrower band of potency. No company is going to intentionally manufacture tablets that are 105% potent and risk that the tiniest variation, say it hits 105.1% potency, could see a whole batch being scrapped. And manufacturers are required to target an exact 100%. (In the past, some companies have made their product super-potent to try to be just within the accepted range when manufactured, and then slowly reduce, with the effect of extending expiry dates. When this was permitted, sometimes tablets reached patients while still super-potent. Because some did this, and some didn't, it was difficult to switch makes. If you went from a make that started more potent, most other makes could appear to be weak.)
And the 90% at expiry date has to be reached even if storage conditions are not optimum. If they have been stored well, for example if much of their time in storage has been in a properly environmentally managed pharmaceutical warehouse, they should be well within the permitted range. Even if relatively poorly stored, but within what the manufacture puts on the product documentation, they should never drop below the permitted range.
I have used potencies of 100% (to represent brand new stock), 97.5% (to represent what I guess is fairly typical) and 95% (which I hope is the lowest likely to be received).
Absorption
Then I have taken the reported absorption of levothyroxine. Typically this is said to be 60 to 80%. However, with IBSA Tirosint and other gel and liquid products, this can be much higher and approach 100%. Absorption below about 60% strongly suggests a serious digestive issue such as gastroparesis.
Potency and absorption can each have a significant impact. And when we combine them it gets worse.
The table below illustrates this. It uses several potencies and
absorptions to illustrate the impact. Nothing is ever this simple and it
should not be taken as a way to look up what is happening to an
individual.
Stomach acidity
There is little good and well-reported information about stomach acidity and levothyroxine absorption. However, there have been reports that things like lemon juice or ascorbic acid, taken close to levothyroxine tablets, can enhance absorption. It is not clear if this also applies to other acidic substances like apple cider vinegar, betaine HCl, etc., which might well have a similar effect regarding increasing stomach acidity.
This raises the question as to whether stomach acidity varies in healthy humans. It does. With a peak level of hydrogen ions around 22:00 and corresponding lowest pH value (that is, most acidic). And a maximum pH at around 09:00 which roughly corresponds to the time at which most product documentation suggests that levothyroxine should be taken.
Circadian rhythm and sleep influences on digestive physiology and disorders
September 2014ChronoPhysiology and Therapy 2014
DOI:10.2147/CPT.S44806
 |
| Figure 1 The circadian pattern of gastric acid secretion across a 24-hour period. |
https://www.tandfonline.com/doi/full/10.2147/CPT.S44806
If stomach acidity is an important factor, that appears to imply that bed-time dosing might be better. That is discussed in another blog:
helvella - Bed-time dosing of levothyroxine
Other Factors
Factors which affect absorption include consumption of many foods, drinks, supplements and other medicines within a relatively short time before or after taking levothyroxine. (Probably one to four hours.) And just taking some medicines at all - especially those that affect acid production in the stomach.
The actual ingredients of tablets also affect absorption. It is suggested that using mannitol (instead of lactose) results in slightly better absorption. Indeed, I suspect that lactose is an ingredient which affects absorption in many of us - even if we are not conventionally lactose-intolerant.
Discussion
We effectively will never be able to find out the potency of our tablets. They could be produced at 100% and only drop to 99.5% after a full two years of storage. Or they could be much worse. Other than in the rare event that the MHRA runs an investigation and publishes the results, will we ever know more.
Nor can we find the percentage we absorb.
Two people could take their 100 microgram tablets. One might absorb 80 micrograms. The other 57 micrograms. And that is from considering just two factors - both of which are entirely feasible.
[i][b]helvella - Levothyroxine absorption[/b]
A short blog discussing the interrelationship of potency and absorption, and stomach acidity and timing.
Last updated 12/02/2025[/i]Link to blog:
https://helvella.blogspot.com/p/helvella-levothyroxine-absorption.html
If you find anything incorrect, misleading, typos, links that don’t work, etc., please let me know. Go to my profile and use the contact details there.
