Most who take levothyroxine (LT4) do so once a day.
Many who take liothyronine (LT3) do so more than once a day. (Though some do prefer once a day dosing.)
Many who take desiccated thyroid do so more than once a day.
From taking LT4 or LT3 to peak blood levels typically takes to to four hours. (Assuming taking on an empty stomach.)
The sharp peak of FT4 and/or FT3 after dosing means that it is hard to draw blood at the precise peak. When sequential blood draws have been performed in research, even a few minutes could make a difference. If only doing a single blood draw, the timing of the peak is necessarily guesswork.
From the sharp peak, both FT4 and FT3 fall away, faster at first, slowing later. The time before the next dose is often called the trough. (FT4 drops more slowly than FT3.)
(The sharpness of the peak is not so clear for FT4. It would be very much more obvious if the origin were shifted to approximately the trough level. But that would be inappropriate.)
At any point from the peak - for many hours - there is an appreciable rate of change. Therefore timing would be expected to affect the result.
Because the rate of change in the trough is low, precise timing is not so important.
For these reasons, it appears that blood draws should be done consistently in the trough. That will achieve maximum comparability of results from one test to the next.
It might be suggested that all results could be interpreted, regardless when the blood was drawn, by applying various "corrections". This has some appeal but we are all different. If we knew our own profiles, we might be able to make some educated guesses. But we don't. So any attempt to correct is based far too much on guesses.
An ideal might be continuous monitoring (as is now being achieved for glucose in diabetes). Next, something like hourly measurements. But neither of these is remotely feasible outside research.
As so often, Tania at Thyroid Patients Canada has already done an excellent article:
Free T3 peaks and valleys in T3 and NDT therapy
November 25, 2019
https://thyroidpatients.ca/2019/11/25/free-t3-peaks-and-valleys-in-t3-and-ndt-therapy/
The following quotes are somewhat old and are far from ideal. However, they do have some interest:
In monitoring patients with hypothyroidism on L-thyroxine replacement, blood for assessment of serum free T4 should be collected before dosing because the level will be transiently increased by up to 20% after L-thyroxine administration (72.). In one small study of athyreotic patients, serum total T4 levels increased above baseline by 1 hour and peaked at 2.5 hours, while serum free T4 levels peaked at 3.5 hours and remained higher than baseline for 9 hours (72.).
Clinical Practice Guidelines for Hypothyroidism in Adults: Cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association
ATA/AACE GUIDELINES| VOLUME 18, ISSUE 6, P988-1028, NOVEMBER 2012
https://www.endocrinepractice.org/article/S1530-891X(20)43030-7/fulltext
DOI: https://doi.org/10.4158/EP12280.GL
Although not recommended, if used, the following common sense recommendations to monitoring might apply. LT3-only therapy should be monitored with measurements of fasting serum TSH levels before the administration of the dose, aiming to target serum TSH within the lower tertile of the reference range, if aiming for TSH values seen in a normal population (54). Although data to support this approach are not available, it would seem reasonable to measure serum T3 levels in the morning (trough) and 2 hours after the administration of the dose (peak), aiming to maintain the values within the reference range; serum FT4 assessment is of no value in this scenario.
Guidelines for the Treatment of Hypothyroidism: Prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement
https://www.liebertpub.com/doi/full/10.1089/thy.2014.0028
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[i][b]helvella - Scheduling Blood Draws [/b]
Factors to consider when choosing time for blood draws.[i]
https://helvella.blogspot.com/p/helvella-scheduling-blood-draws.html
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