Assessing Your T4 to T3 Conversion Capability: Are You A Good Enough Converter of T4? Do you have enough T3?

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:

Metabolic Category

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.

Best wishes,


Paul Robinson

Paul Robinson is a British author and thyroid patient advocate. The focus of his books and work is on helping patients recover from hypothyroidism. Paul has accumulated a wealth of knowledge on thyroid and adrenal dysfunction and their treatment. His three books cover all of this.

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  1. Alane W on 14th May 2022 at 3:38 am

    So I have a question. I calculated my conversion ratio and I got .509. So that means I am a good converter. I am on T4 therapy. My free t3 is 2.5, My free t4 is 1.40, My RT3 is 15.9, My TSH is 2.680. So then if I am a good converter why would my T3 be so low? How do I solve this problem?

    • Paul Robinson on 14th May 2022 at 10:15 am

      Alane, do make sure that both the FT4 and FT3 were in the same units. They have to be in the same units for this calculation to work.

      One of the big mistakes doctors make is to use TSH to limit how much Levothyroxine a patient is given. If the person is not hyper and they have low FT3, the T4 meds ought to be increased further until 1) FT3 is higher and 2) symptoms and signs improve. Even if TSH gets very very low this is perfectly safe as the research suggests.

      Best wishes, Paul

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