It looks like there is no direct conversion. It would be interesting to know.
If someone has NO thyroid function, they would need the 100 nmol T4 and 5 nmol T3 per day. So I think that would be higher than a dose of 100 mcg/day.
I suppose that is why getting the right dose is part guessing game.
Yeah, there must be a range of normal and that is the average.
I also had this thought that I wrote down to ask the endo:
The thyroid secretes t4 mostly and some t3. If the thyroid is impaired then certainly it may not be producing all of the t3 it used to. If so, even if you convert the maximun amount of t4 to t3, you may still be short of t3.
Since t3 is the active hormone, then it seems possible (or likely) that we would be partially deficient in t3 and it probably makes a difference in how we feel.
This is getting pretty technical, but I found this in Thyroid Manager:
"That most plasma T3 is derived from peripheral conversion of T4 is supported by the fact that normal plasma T3 levels are obtained in athyreotic patients treated with sufficient T4 to achieve high normal plasma T4 levels. A recent study has shown that administration of T4 replacement doses to hypothyroid rats achieving normal plasma T4 levels results in subnormal plasma T3 levels not only because of the lack of T3 secretion but also because of a decreased T3 production by D1 in peripheral tissues, since this enzyme is under positive control by T3 itself (5). This study as well as a recent clinical study (6) suggest that thyroid hormone replacement of hypothyroid subjects is done best by treatment with a combination of T4 and T3 (preferably as a slow-release formulation) in proportion to their normal thyroidal secretion."
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