This is what I've generally round in following that "trail," but it's one that doesn't go very deep, from what I've found.
Have you found any recent work looking at carnitine (used alone) and blood markers?...
It's surprising to me from regulatory standpoint that there would be no feedback inhibition or cellular down regulation of the inhibitor effects of carnitine on T3 action, but taking carnitine in isolation is a modern phenomenon. OTOH, the content of carnitine in meat can be pretty high (
https://pubmed.ncbi.nlm.nih.gov/23879010/ Suggests 637mg in 100g of dry matter kangaroo meat, which might be about ~600mg in .4kg or 1lb of fresh meat.)
I suspect the issue of testing this in isolation in cases of thyroid storm or hyperthyroidism of various causes is ethical, as I see other studies using a combination carnitine with drugs that interfere with thyroid output / metabolism (so the effect of L-carn.)
So, would be interested to know from anyone who's got blood work (T4, T3, free, reverse, etc.) with and without high dose L-carn.
In the attached study there was no effect on blood markers in two of three subjects, but subject C seemed to show some reduction in thyroid hormone ("protein bound iodine") and perhaps metabolic rate. There was no mention of diet here, so it could of course be that with reduced hyperthyroid symptoms Subject C's lifestyle changed (less anxious and moving around less... perhaps a nervous eater previously, etc. so that subject's response was reflective of some normal thyroid metabolic regulation.)
This was interesting, too - suggests even a different etiology of the hyperthyroidism in this subject...
"Subject C did not completely follow the
above pattern. Indeed, there was an initial
depression of PBI along with the clinical
improvement; however, his PBI remained
depressed throughout the duration of
carnitine administration. Following the
administration of placebo, a period of
eight weeks elapsed before the signs and
symptoms of hyperthyroidism recurred.
At this point his PBI was elevated to 9.3
micrograms/IOO ml. Since subjects A
and B had a recurrence of their clinical
hyperthyroid manifestations by the end
of the second week, it seems unlikely that
the delay in subject C was due to a variation
in drug effect. The clinical and
laboratory course of subject C can be ex-l
plained if the patient had undergone
spontaneous remission while on carnitine.
The BMR was not significantly lowered
from the pretreatment level in subjects
A and B; however, there was a definite
elevation above the pretreatment levels by
the end of the placebo period. In subject
C, the BMR was lowered after carnitine
administration and remained so until the
eighth week of placebo administration."
-------------
Then there's also the issue of whether there is indeed a relevant effect of long term carnitine on the thyroid hormone driven aspects of metabolism that might undo the impact on fat oxidation over time. Plus the interaction of L-carn on thyroid action in those who are hyper vs. euthyroid. Perhaps the reduction in hyperthyroid symptoms of using L-carn does not reflect the extend of genomic inhibitory actions in those who are not hyperthyroid.
I've also seen that hyperthyroidism can deplete tissue levels of L-carnitine (can find that again I imagine for those interested) so there's another interaction to and possible mechanism at work in treating hyperthryoidism with L-Carn, as well as a reason for those who go high on Thyroid "supplementation" to ensure they've got L-carn in the mix (dietarily a/o via supplementation).
-S
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