The Love-Hate Relationship With Levothyroxine

[PR4NOW]

by Angela M. Leung, MD, MSc October 25, 2018 An add on to the posts by Diogenes.

The comments to the post are quite interesting. This is a Medscape article but to join is still free I believe. PR

medscape.com/viewarticle/90...

[helvella Administrator Thyroid UK]

Slight conflict. The paper linked says:

Although more costly, Tirosint is a gel capsule of synthetic levothyroxine that contains no sugars, dyes, alcohols,

But Tirosint is based on glycerol which:

For human consumption, glycerol is classified by the U.S. FDA among the sugar alcohols as a caloric macronutrient.

The paper also says:

For example, an individual who has been taking 112 µg of synthetic T4 once daily and wants to try incorporating T3 would be prescribed 5 µg of synthetic T3 twice daily combined with 100 µg of synthetic T4 to reach a total of 110 µg of thyroid hormone per day, which is as close to the previous weight-based dose as possible.

Regardless any other factor, one molecule of liothyronine is significantly lighter than one molecule of levothyroxine. 100 micrograms of levothyroxine would actually convert to about 86 micrograms of liothyronine. Ironically, the 12 microgram reduction in levothyroxine which is casually suggested actually is very close to 10 micrograms of liothyronine. But I very much get the impression that fact is not appreciated.

[jimh111 in reply to helvella]

It's not the molecular weights that matter but the fact that in tablet form liothyronine is about 3x as potent at levothroxine. Thus 10 mcg of L-T3 equates to about 30 mcg L-T3. How did this person get an MD and MSc and why wasn't the article peer review?

The point about pregnancy and T3 is an enigma. As far as we know little T3 crosses the placenta so logically pregnant women should be on T4. However, many women on T3 have had successful pregnancies.

[PR4NOW in reply to jimh111]

Jim, I've always found it curious that so little T3 seems to cross the placenta. I wonder if they will eventually find that it is like "the lymph system stops at the neck". PR

[helvella Administrator Thyroid UK in reply to jimh111]

But he uses the aim, as quoted, to reach a total of 110 µg of thyroid hormone per day, which is as close to the previous weight-based dose as possible !

I am simply pointing out that a complete change from T4 to T3 on that basis would actually be an increase in dose in molar (number of molecules) terms by around 15% For which there seems to be absolutely NO basis.

On your suggested three-to-one conversions, adding 10 micrograms of liothyronine should reduce levothyroxine by 30 micrograms. (I accept there are arguments that the reduction in levothyroxine could be inappropriate. But following his and your suggested rationales, it doesn't make sense.)

[jimh111 in reply to helvella]

There's a weight based recommendation of 1.6 mcg levothyroxine per kg. This is just a typical dose needed to fully replace hormone secreted by the thyroid and bring the TSH within its reference interval. It's just a rough starting point and not meant to be a target or imply any sort of precision.

This doctor's aim of retaining the same weight of tablets is bonkers. On that basis 110 mcg of school room chalk would be as beneficial! As you point out T3 is a little lighter than T4 so you would end up with more T3 molecules. More importantly T3 is more potent than T4, about 3x as potent in tablet form ncbi.nlm.nih.gov/pmc/articl... . So, for patients on moderate doses of L-T4 you could subsitute L-T3 on a 3:1 basis, provided clinical response is given precedence.

She also asserts 'although DTE has been used for much longer than synthetic formulations, there is a lack of high-quality, controlled studies to demonstrate that DTE is superior to synthetic levothyroxine' which is really saying there are no high-quality studies to show L-T4 is as good as NDT.

I think we would both run a mile from this doctor.

[helvella Administrator Thyroid UK in reply to jimh111]

Typo time! 1.6 rather than 6, I think.

As for running, most certainly.

[jimh111 in reply to helvella]

1.6 I've edited it thanks.

[Angel_of_the_North in reply to jimh111]

A peer is just someone who is as ignorant as you are ...

[jimh111 in reply to Angel_of_the_North]

... and usually someone whose paper you have approved. It's sad, we need good honest peer review.

[Angel_of_the_North in reply to jimh111]

We needs reviews by those who know more: "betters", not equals!

[PR4NOW in reply to helvella]

Rod, if you have the time read some of the (73) replies to the article. They illustrate the variance here in the US about how to treat hypo. PR

[helvella Administrator Thyroid UK in reply to PR4NOW]

Good to see a similar level of misunderstanding as in the UK! Albeit with greater acceptance of liothyronine.