Can Adding Some T3 to Mostly T4 (NDT or Levothyroxine) Clear rT3 (Reverse T3)?

I was recently asked by a friend whether it was true that by adding some T3 medication to their main thyroid medication of natural desiccated thyroid (NDT) or thyroxine (T4) that this would clear reverse T3 (rT3).

The answer to this is very simple. No, it won't clear rT3 if someone just adds some T3 to NDT or T4 medication - the basic science is very straightforward.

Let me explain.

Clearing rT3 means reducing rT3 to a negligible or zero level. If someone is taking NDT (which is mostly T4 with some T3) or taking T4 medication then there will always be on going conversion of T4. The T4 will be converted partly to T3 and partly to rT3 and there isn't anything that can be done to stop that happening. 

However, there are some interesting things to be aware of.

The amount of NDT medication or T4 medication being taken is significant. There are two interesting cases.

Case 1. If there is enough NDT or T4 being taken, such that TSH is fully suppressed to zero, then the conversion of T4 to T3 will already be minimised, i,e. it will be as low as it can get due to TSH. RT3 in this case will be produced at its maximum rate. Please see the blog post to explain this.  In this situation, with zero TSH, any T3 medication that is added will increase FT3 somewhat. So, in this situation the FT3 / rT3 ratio should improve. However, rT3 will not go down, i.e. it won't clear.

Case 2. If the person is not taking enough NDT or T4 medication to suppress TSH to zero, then adding extra T3 will further lower TSH. This lowering of TSH will actually incur a worsening of the T4 to T3 conversion. So, even though the patient is now adding some T3 meds, their FT3 may not go up much and it could even fall. The other aspect to this is that as the conversion of T4 to T3 worsens there will be more rT3 created, i.e. rT3 will actually go up. Whether FT3 / rT3 ratio improves is a bit of a lottery, as it depends on how much TSH suppression is created by the addition of T3 and how much actual T3 is added. 

In no case of adding some T3 medication to a mainly NDT or T4 regime does rT3 actually clear and reduce towards zero. In some cases a better FT3 / rT3 ratio may result. In others the FT3 / rT3 ratio may not improve, and for a few it could actually worsen. 

We know from experience that some patients do benefit from adding some T3 to their T4 meds or to their NDT meds. I am not arguing with this at all. I am just explaining the effect upon rT3 and explaining that rT3 cannot be 'cleared' by simply adding some T3 to NDT medication or T4 medication. 

We also know from experience that regardless of any particular FT3 / rT3 ratio that some number of patients just don't get well until they actually do begin to clear rT3, and this can't be achieved on a mainly T4 based regime (NDT or thyroxine). True clearance of rT3 requires either all T3 based medication OR, if someone only requires most of the rT3 to be cleared, then signficantly reduced NDT/T4 medication and multi-dosing of enough T3 medication.

Interestingly, the blog post referred to above, which shows the effect of suppressed TSH on conversion ration of T4 to T3 ( applies even if TSH is low due to a malaadjusted TSH or even a hypopituitary issue that affects TSH. The liver does not care about the reason for the low TSH is simply uses TSH as parts of its input to decide upon how much T4 to convert to T3 and how much to convert to rT3.  This may go some way to explaining why some patients with hypopituitary issues or other issues leaving them with low TSH may have more problems processing thyroid medication that is mainly T4 based.

The other important thing mentioned in the blog post referred to in the link is that it can take a week or so for the TSH to adjust when any small amounts of T3 are added. Then if there is a conversion rate adjustment there may be a small lag as FT3 levels may fall and rT3 levels may rise. The net result is that it is quite common for people who add a little T3 to actually feel the effects of the additional T3 immediately and think that they are really getting a good result. This frequently turns to disappointment as the positive effects wear off because of the re-adjustment of conversion rate. Again, this isn't to say that some people don't actually get a benefit from adding a little T3 to T4 or NDT meds - because some do. It just isn't a guaranteed result by any means for at least all the above reasons.

This is a bit complex but for those that are interested I hope it helps.

Best wishes,