Yikes! I have been searching on Google for an accurate description of the drug liothyronine. So far, nearly all of the sites describe it this way:
"Liothyronine is a man-made form of a hormone that is normally produced by your thyroid gland to regulate the body's energy and metabolism. Liothyronine is given when the thyroid does not produce enough of this hormone on its own."
Is there any online site considered to be a reputable source that tells users that the thyroid gland only makes a small amount of the hormone, that most is made in the liver by removing one of the iodine atoms from the T4 (thyroxine) molecules? And also that accurately tells us that liothyronine is given when the body doesn't convert enough (not that the thyroid doesn't produce enough), or when treatment with levothyroxine fails to alleviate all hypothyroid symptoms? Link please.
So far I have looked at
Drugs.com
webmd.com
medlineplus.gov
uofmhealth.org
pubchem.ncbi.nlm.nih.gov
goodrx.com
rxlist.com
NOTE: This last site at least discusses the structure of the molecule and includes this text: "Cytomel (liothyronine sodium) Tablets may be used in preference to levothyroxine (T4) during radioisotope scanning procedures, since induction of hypothyroidism in those cases is more abrupt and can be of shorter duration. It may also be preferred when impairment of peripheral conversion of T4 to T3 is suspected."
I have been unable to get to mayoclinic.org. The web site is down
At the bottom of my search results, I found this link:
As far as drug companies are concerned that is the correct description, it is a man made thyroid replacement hormone. It might be worth looking up triiodothyronine, the name of the naturally occurring hormone for more information on function and quantities produced in health.
I agree that it is a man-made hormone. My problem was with this: "Liothyronine is given when the thyroid does not produce enough of this hormone on its own."
When all the drug trials for liothyronine were done it was given instead of levothyroxine, for people with with primary hypothyroidism. It was for a long while the only form of thyroid replacement hormone available for intravenous administration and it is still the drug of choice in myxoedematous coma. Therefore when it was licensed, it was approved as a thyroid replacement hormone. There are many people who take t3 only and do well. It is only since the removing of ndt from the formulary that interest has grown in combined synthetic hormone replacement. As I’m sure you are aware there is a paucity of robust clinical data on combined treatment and as it not perceived to be a money spinner by the pharmaceutical companies that is likely to continue.
My theory for this is that the research has concentrated on trying to come up with a universal ratio of T4 to T3 for dosage, rather than using blood levels of FT4 and FT3 (how far through the range, aiming for at least the upper half) as a guide to how well the dosage they are trying for any particular patient is working. I'd love to see such a study that would also use endpoints such as perceived energy level, weight gain or loss, and quality of life.
This paper makes some attempt to describe thyroidal versus peripheral T3 production, but as an accurate guide I don't recommend it, only as a document supporting direct thyroidal T3 production
Am J Physiol 1990 Apr;258(4 Pt 1):
E715-26. doi: 10.1152/ajpendo.1990.258.4.E715.
Thyroidal and peripheral production of 3,5,3'-triiodothyronine in humans by multicompartmental analysis
A Pilo 1 , G Iervasi, F Vitek, M Ferdeghini, F Cazzuola, R Bianchi
In an older study that was listed underneath the abstract you linked, I found this
"In the control group, 72.0 +/- 3.6% of the circulating T3 was produced by 5'-monodeiodination of T4 in the peripheral tissues, and 28.0 +/- 3.6% of the circulating T3 derived from direct thyroidal secretion. The sick euthyroid patients showed a significantly smaller proportion of circulating T3 deriving from peripheral conversion of T4 (52.5 +/- 3.9%; P less than 0.025)."
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