Is is OK to have Zero/Suppressed TSH on Thyroid Treatment?

I was just asked the question whether it is OK to have Zero (or suppressed TSH) when on T4/T3 treatment (or any thyroid treatment).

This was my answer:

The research is clear.

A suppressed TSH when on thyroid treatment is NOT a concern at all, AS LONG AS 1) the person has no hyperthyroid symptoms and 2) FT3 is not above the range (but this has caveats).

The above statement is true on T4 and I will provide the research references below to back it up.

What about T4/T3 or T3-Only therapy?

On ANY T4/T3 based treatment, I believe FT3 can creep up to and over the range also. There is a very clever but a little complicated explanation for this.

It goes like this:

People who are healthy, or on T4 medication, have on-going T4-to-T3 conversion happening within their cells. Much of this intra-cellular-T3 that is converted from T4 remains in the cells and is used there. Some of it gets returned to the bloodstream. This is IMPORTANT. The FT3 reference range for the population is created by measuring blood levels of FT3. So, for people who are healthy, or on T4-therapy, they have the FT3 that can be measured and EXTRA FT3 in the cells that CANNOT BE MEASURED (unless we dissect them - I don't think they'd want that! I know I wouldn't). So, the FT3 reference range does not account for the intra-cellular extra FT3.

Now, as soon as you use T4/T3 treatment or NDT, you have extra T3, but usually less T4 than you previously had (and certainly it won't be converting as well due to the effect of the T3 on suppressing TSH more - which lowers conversion rate). So, the FT3 measured in blood is almost certainly going to be higher for someone on T4/T3 therapy AND there will be less intra-cellular T3 being converted from T4. The net result is that T4/T3 therapy patients are much more likely to get close to or over the FT3 reference range - it wasn't designed for them, so no real surprise anyway.

The situation gets a whole lot worse for people on T3-Only. They have NO intracellular T4-to-T3 conversion at all. Their FT3 level is often well over the range - to compensate for no intra-cellular T4-to-T3 conversion. Our UK reference range for FT3 is typically 3.3-6.6. My FT3 when tested is about 9. I have zero FT4 and zero TSH. I am NOT hyperthyroid. This makes tons of sense when you understand the real endocrinology.

Unfortunately most doctors do NOT understand the fundamentals.

I hope this helps to explain it.

It is described in ALL my books now and the research and many other research findings are in The Thyroid Patient's Manual.

Here are some of the research references from Chapter 14 of The Thyroid Patient's Manual.

1. Symptomatic Relief is Related to Serum Free Triiodothyronine Concentrations during Follow-up in Levothyroxine-Treated Patients with Differentiated Thyroid Cancer - Larisch, Midgley , Dietrich and Hoermann. See: https://www .thieme-
connect.de/DOI/DOI?10.1055/s-0043-125064
(This paper clearly proves that FT3 concentrations are the most important in clinical decision making, as they are most closely linked to residual hypothyroid symptoms in T4- Only treated patients. It also shows that in-range TSH is not sufficient for symptom relief.).

2. Homeostatic equilibria between free thyroid hormones and pituitary thyrotropin are modulated by various influences including age, body mass index and treatment - Hoermann R1, Midgley JE, Giacobino A, Eckl WA, Wahl HG, Dietrich JW, Larisch R. See: https://www.ncbi.nlm.nih.gov/pubmed/24953754
(This is a great paper, although complex. It shows that it is the relationships of the thyroid hormones, and how they adjust during treatment, that counts. It also makes it crystal clear that TSH should only have a supporting role in the assessment process during treatment.)

3. Recent Advances in Thyroid Hormone Regulation: Toward a New Paradigm for Optimal Diagnosis and Treatment - Hoermann, Midgley, Larisch, Dietrich. See: https://www .frontiersin.org/articles/10.3389/fendo.2017.00364/full
(This paper talks about the need for a new paradigm of thyroid treatment that accepts that the relationship between TSH and thyroid hormones are individual, dynamic and can adapt, i.e. the current practice of simply looking at numbers that do or do not fit in the population ranges is not sufficient).

12. Thyroid hormone replacement - a counterblast to guidelines - Dr A.D. Toft. See: http://www.rcpe.ac.uk/sites/default/files/jrcpe_47_4_toft.pdf

13. Consensus statement for good practice and audit measures in the management of hypothyroidism and hyperthyroidism - M P J Vanderpump, J A 0 Ahlquist, J A Franklyn, R N Clayton, on behalf of a working group of the Research Unit of the Royal College of Physicians of London, the Endocrinology and Diabetes Committee of the Royal College of Physicians of London, and the Society for Endocrinology See: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2351923/pdf/bmj00557-0041.pdf

I hope people find this interesting - it is very important stuff.

Best wishes,

Paul