Application of the T3/rT3 Ratio, in managing IH

T3, reverse T3 (rT3) and the T3/rT3 ratio: The normal ranges

The normal range for T3/rT3 is “greater than 20”: the optimal range is greater than 24.

The normal T3 range is 3.1–6.1 (? 5.8) Pm/L (the T3 range was originally quoted as 3.1–6.1 pmol per litre: this was changed to 2.8–5.8 pmol per litre in 2021, but in my opinion the change was unfounded). I regard the correct range of T3 as 301-6.1.

There is no “normal”range for rT3: reverse T3 is not regarded as a metabolite, because it is thought to have no function (although it does up-regulate Deiodinase 3, thus encouraging degradation of T3 in the cells to T2). rT3 however, as a “marker”, is of real diagnostic value, because its level gives an indication of the degree to which T4 is being converted into reverse T3 in our cells.

An elevated rT3 signifies reduced production of T3, from T4.

Calculating T3/rT3

rT3 values are reported in nanograms/DeciLitre, while T3 is reported in picomoles/Litre. Therefore, to calculate the T3/rT3 ratio, we first convert T3 values into Ng/DL, then divide the T3 value by that of rT3. A normal result is >20 and an optimal result is >24.

Table, for calculation of T3/rT3 ratio, to diagnose IH

This valuable, Time-saving table is copied
with permission, from Cameron Sutherland, PhD.
T3 (in picomoles/L) values are in the light blue column.
T3 (converted to nanograms/DL) occupies the yellow column.
The light blue horizontal numbers (1st row) are reverse T3 values.
Normal, “euthyroid” T3/rT3 values are shown in the pink area of the columns.
Subnormal T3/rT3 values, diagnostic of IH, are in the white area of the columns .

Interpretation of T3/rT3 values, targets for T3 and rT3 and dose schedule for S-R T3

Assuming the normal range of T3, to be 3.1 – 6.1, a euthyroid level of the T3/rT3 ratio to be 20 or more,
and a “target” range for T3 as 4.1 – 5.5, we can derive an arbitrary, “desirable” range for rT3, via the following logic:

Basic facts (vide supra)
At a T3 of 3.1, the patient is euthyroid if the rT3 is less than 11.
If T3 is between 4.1 and 4.3, the patient is euthyroid if the rT3 is up to 13.
If T3 is between 4.4 and 5.5, the patient is euthyroid if rT3 lies between 14 and 17.
At a T3 of 6.1, the patient is euthyroid if the rT3 is less than 20.
If rT3 exceeds 20, euthyroidism is unattainable unless T3 is in the hyperthyroid range.

Observed facts
(1) On a practical basis (based on my experience of more than 200 cases), raising the FT3 to greater than 5.0 pmol/litre with S-R T3 results in serum rT3 levels in the range of 7–10 ng/decilitre.
(2) In a few instances among my 200 patients, S-R T3 administration resulted in T3 levels of 6.2 – 8.0.
In most cases, the only side effects were a feeling of being “high” and intensification of visual acuity: only 3 of these patients experienced “antsiness”, an elevated pulse rate or other hyperthyroid symptoms.
In these 3 cases, reduction of the SR T3 dose resulted in an uneventful reduction of FT3 to less than 6.2 and cessation of hyperthyroid symptoms.

Reasonable, though arbitrary, protocol
I regard the desirable range for T3 as 4.4 – 5.5: and the target level for rT3 as less than 13. These targets are easily achievable, via ad. hoc. titration of slow-release T3 (SR–T3) dose, beginning at 10 µg per day and increasing weekly in 5 µg (per day) increments until the T3 is greater than 4.4, at which point the patient’s hypothyroid symptoms are reassessed, prior to considering a change of dose.
The dose of S-R T3 is increased if the serum FT3 is less than 4.4 and reduced, if it exceeds 6.0.
If the hypothyroid symptoms fail to subside, the S-R T3 dose may be increased until the FT3 rises to 6.1.