From time to time, a thyroid patient asks me the question: how can they have a reasonably high level of rT3 or FT4 when they take T3-only medication?
Let me be very clear. T3 cannot under any circumstance convert into rT3. T3 and rT3 are what is known as isomers. They have the same number of elements making up their molecules but the elements are connected in different ways.
T4 can be converted in the body into either T3 or into rT3. But once T3 or rT3 is made, the body has no mechanism to take elements from a T3 or rT3 molecule and reorganise them together in a different way. So, rT3 cannot be converted to T3. Equally, T3 cannot be converted to rT3. This is impossible. It would be a bit like saying we could take all the cells that make up a tiger and rearrange them to create a lion. It just cannot be done.
So, if someone is on T3-only, and they still have some measurable FT4 and rT3 there are only a few ways that this can be caused:
1) There is still some circulating T4 from any T4 medication that was being taken prior to the person going on T3-only. Normally, after stopping all T4 medication and being on a TSH suppressive dose of T3 medication, it takes a further 12 weeks to clear all the FT4. A TSH suppressive dose of T3 might be 40 mcg for some people and well over 100 mcg for others.
2) The person is not on enough T3 to cause TSH to be zero over the full 24-hour hours. Sometimes TSH can rise at night – so testing TSH in the daytime may not detect that TSH is not suppressed. If TSH is not zero all the time and the person still has any thyroid tissue left (even after a thyroidectomy), then some T4 will be produced, much of this T4 may go into rT3. In some cases, after a total thyroidectomy that shows there is no thyroid tissue left, new nodules can be produced – hence the need for regular check-ups.
3) In rare cases, someone might have a neuroendocrine tumour. These are benign hormone-producing tumours. Some neuroendocrine tumours produce hormones and some produce neurotransmitters like serotonin. Even though they are not cancerous, they can be incredibly difficult to diagnose and detect and require sophisticated scanning techniques. In rare cases, someone could have a neuroendocrine tumour that produces T4 (which can then be converted to rT3) even if the person has no thyroid gland. This is included here for completeness only as it is not common.
4) Lab error. This is unusual but it actually does happen. Sometimes the results of a thyroid lab test may make no sense. If this is the case, it may be worth questioning the laboratory staff about the results. I have definitely heard of cases when either rT3 or other lab test results have been mistakes and the test has needed to be repeated.
The other obvious conclusion from all of this is that if a thyroid patient is on a T4/T3 combo and has continued high rT3, the best solution is to reduce the T4 content of the combo and possibly to increase the T3 content (based on symptoms & signs and lab test results). It is always the T4 that is the source of the rT3.
I hope this helps those people who have had this question.