Dr Tania S. Smith, of Thyroid Patients Canada, wrote an article on assessing conversion ability from T4 to T3 in late 2018. It is extremely useful and I think it might help some thyroid patients to understand why they are having problems.
Dr Smith’s article is entitled: “Are you a poor T4 converter? How low is your Free T3?”
Here is the link to the article on The Thyroid Patients Canada website:
Dr Smith’s article refers to this research paper:
Variation in the biochemical response to l-thyroxine therapy and relationship with peripheral thyroid hormone conversion efficiency. Endocrine Connections, 4(4), 196–205.
Midgley, J. E. M., Larisch, R., Dietrich, J. W., & Hoermann, R. (2015).
Here is a link to the research paper:
One of the findings in the research study above that Dr Smith highlights is that it is possible to assess conversion capability in those people on no thyroid medication and in thyroid patients on Levothyroxine only (T4 only thyroid medication).
You can read both the original research paper by Hoermann, Midgely et. al. and Dr Smith’s paper for the details.
However, based on both the above, there is a relatively simple way that may help many of you to assess how well you process your T4 thyroid hormone and how efficiently it is being converted to T3.
Note: this method only works for people on no thyroid meds at all or those patients on LT4 monotherapy. In addition, both the FT4 and FT3 results need to be from the same laboratory and converted into the same units (below FT4 and FT3 are in pmol/L, but the important thing is that they are in the same units). It is also important to realise that this is also not the same as conversion ratio or what proportion of FT4 is being converted into FT3. It is just a way of categorising how effective the person converts from T4 into T3.
Simple Method of Assessing Conversion Capability:
Poor Converters on T4 Monotherapy
Intermediate Converters on T4 Mono
Good Converters on T4 Mono
FT3 in pmol/L Divided by FT4 in pmol/L
I hope you found this information useful. There was little point in me repeating the information contained in Dr Smith’s article and the research paper cited, so please read them for more details.