The issue
When we take levothyroxine (LT4) or liothyronine (LT3) - or a combination of LT4 and LT3 - this has a fairly rapid effect on Free T4, Free T3 and TSH levels.
The peaks of FT4 and FT3 are typically somewhere around two to three hours after ingestion - though this could be earlier or later depending on the individual and the formulations of LT4 and LT3.
The graphs are from a very small sample but correspond quite well with other similar trials. We can expect similar times across most of us.
The peaks/troughs shown should be seen as illustrative rather than values that would be expected in any individual. In each of us, they could be earlier or later, sharper, and more pronounced, or less so.
Free T3 profiles for 24 hours after administering either LT4 or LT4/LT3 in 10 patients with hypothyroidism with mean values of (a) fT3 shown.
Free T4 profiles for 24 hours after administering either LT4 or LT4/LT3 in 10 patients with hypothyroidism with mean values of fT4 shown.
TSH profiles for 24 hours after administering either LT4 or LT4/LT3 in 10 patients with hypothyroidism with mean values of TSH shown.
https://eref.thieme.de/ejournals/1439-3646_2007_04#/10.1055-s-2007-973071
Oral Solutions (Liquids)
Very
clearly, liquid products (oral solutions and gel caps) have
significantly different absorption profiles. The important point is that
taking any thyroid hormone affects these three measurements rather than
exactly when the peak occurs.
Discussion
When a
blood test is done it will be affected by having taken thyroid hormones.
The peak - with its sharp rise, brief peak, and slower fall - means it
is impossible to measure in the real world. Thirty minutes later, and
the result could be very different. Or take a formulation which is
slower (or faster) in getting absorbed.
The graphs might also be somewhat different if we take our thyroid hormones at different times of day. And whether we have eaten thus raising blood glucose and changing insulin, etc., even if we have allowed plenty of time for the food not to directly interfere with absorption.
Issues such as delayed gastric emptying/gastroparesis will also have a potentially major impact on both size of impact and timing.
This discussion ignores the natural rhythms of FT4, FT3 and TSH that we would have if we didn't take any thyroid hormones!
Fudge Factor
The combination of these individual variations and the
various formulations available make it impossible to adjust for these
peaks/troughs in any clinically useful way. A GP or endocrinologist
might be accept that the results have been affected by taking thyroid
hormones, but there is no magic formula they can apply to see what they would
have been if we had not done so.Note on TSH
The change in TSH is much faster than many assume. But the issue with TSH measurement is that it adjusts not just within the short time discussed here, but over days, weeks, even months. If your TSH started out at 5, and dropped to 3 by taking a single dose, it doesn't mean that would still be the case weeks later. By then your TSH might have fallen to 3 at its highest and end up at 1 shortly after your daily dose.
Adjusting your dose by TSH would be misleading if based on a TSH measurement shortly after your daily dose. At 1, that might seem fine. But at 3, it indicates an increment in dose is required.
This is not endorsement of using TSH alone for dose adjustment. The fullest possible picture is required.
What to do?
In my view, the only viable approach to this issue is to do our best to have our blood drawn for tests in the trough period when the peak has disappeared.
For LT4/FT4 that would be around 18 to 24 hours after taking LT4.
But for T3/FT3 waiting that long can see a somewhat artificial dip. Around 12 hours after taking LT3, maybe 9 to 15 hours, seems to reach a stable plateau.
For combinations, whether taking both LT4 and LT3, a combination product or desiccated thyroid, it might be best to treat it as LT3 but understand that FT4 is probably slightly higher than it would be if you delayed it.
If you multi-dose, you have to make the best guess you can from the information. I doubt we will ever see a protocol which works well for multi-dosing.
Other Links
As you might imagine, this topic has been discussed many times. One post has been very popular and you might care to read that.
Suggested Thyroid Hormone Test Timing Protocols
https://healthunlocked.com/thyroiduk/posts/149327184/suggested-thyroid-hormone-test-timing-protocols
If you wish to link to this page on HealthUnlocked, copy the entire dark red text below and paste into a post or reply.
[i][b]helvella - FT4, FT3 and TSH after taking thyroid hormone [/b]
A discussion about how taking thyroid hormones affects blood tests.
Last updated 18/07/2024 [/i]
https://helvella.blogspot.com/p/helvella-ft4-ft3-and-tsh-after-taking.html
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