These first two papers can be used to prove that on T4 monotherapy , a dose that is enough to 'satisfy' the hypothalamus /pituitary (ie. lowers TSH to the bottom end of the ref range) , does not necessarily provide 'enough' T3 to all the other tissues... some other tissues can remain hypothyroid despite TSH being in 'normal' range .
Just because the pituitary thinks that dose is 'enough' ... the heart / lungs/ muscles etc may not have 'enough' .
So some people need to take more T4 (to get enough T3 to those tissues) in order to feel well , and they then end up with a below range TSH and in a constant argument with their GP (who believes their pituitary's opinion on the matter).
( links to both were taken from this : bmcendocrdisord.biomedcentr... Time for a reassessment of the treatment of hypothyroidism John E. M. Midgley, Anthony D. Toft, Rolf Larisch, Johannes W. Dietrich & Rudolf Hoermann ~2019)
jci.org/articles/view/118353 Replacement therapy for hypothyroidism with thyroxine alone does not ensure euthyroidism in all tissues, as studied in thyroidectomized rats.
H F Escobar-Morreale, M J Obregón, F Escobar del Rey, G Morreale de Escobar (1995)
thelancet.com/journals/land... New insights into the variable effectiveness of levothyroxine monotherapy for hypothyroidism
Elizabeth A McAninch ,Antonio C Bianco (2015)
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A below range or supressed TSH in a patient taking Levo is usually taken to mean the patient is overmedicated/ Thyrotoxic (ie. the same as it means in people not taking any replacement thyroid hormone)
The following papers can be used to show why TSH level doesn't mean quite the same thing once on Levo. ~ fT4 is relatively higher / fT3 is relatively lower and TSH is relatively lower in patients on Levothyroxine monotherapy :
a previous reply from diogenes to someone else : "Your GP should realise that in T4 therapy a below-reference TSH doesn't often matter. Indeed this downloadable paper shows that in therapy TSH as low as yours (0.06) can still express the body as euthyroid. You could download it and show its basic conclusions."
(diogenes is an author in some of the papers below [JEM Midgley], also advisor to ThyroidUK ,and the inventor of one of the methods used for fT4/fT3 tests)
ncbi.nlm.nih.gov/pmc/articl... Biochemical Markers Reflecting Thyroid Function in Athyreotic Patients on Levothyroxine Monotherapy Mitsuru Ito, Akira Miyauchi, Mako Hisakado, et al. (2017)
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onlinelibrary.wiley.com/doi... Homeostatic equilibria between free thyroid hormones and pituitary thyrotropin are modulated by various influences including age, body mass index and treatment Rudolf Hoermann, John E.M. Midgley, Adrienne Giacobino, Walter A. Eckl, Hans Günther Wahl, Johannes W. Dietrich, Rolf Larisch (2014) This study proves that once on Levo, the TSH level is shifted lower , and the fT4 is shifted higher ~ relative to the same amount of fT3. Therefore explaining why patients often need a lower than 'normal range' TSH to feel well on just levo. ( they need a higher fT4 to get the same amount of fT3 ~ and in order to get the same amount of fT3 they have a lower TSH)
fig. 2 ~ a graph showing the lower TSH / higher fT4 / same fT3 after levo ~ is accessible via the 'Supporting Information' dropdown at the end of the summary.
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ncbi.nlm.nih.gov/pmc/articl... Increased fT4 concentrations in patients using levothyroxine without complete suppression of TSH (Heleen I Jansen, Marijn M Bult, Peter H Bisschop, Anita Boelen, Annemieke C Heijboer,and Jacquelien J Hillebrand 2023)
"Furthermore, a higher fT4/fT3 ratio was seen in patients using L-T4 compared to healthy controls (12, 21, 22). Hypothyroid patients receive L-T4 supplementation which replaces T4 only and thus these patients partially lack the active thyroid hormone triiodothyronine (T3) derived from the thyroid gland. This suggests a need for increased conversion of T4 into T3 in the peripheral tissues to reach adequate tissue T3 concentrations (21), resulting in the need for a higher L-T4 dosage to obtain a normal, or even somewhat lower, concentration of T3 (22, 23). Moreover, TSH is more sensitive to changes in T3 than in T4, clarifying the lack of negative pituitary feedback to increased fT4 concentrations leading to not (completely) suppressed TSH. This phenomenon may be more prominent in patient groups that are characterized by an even greater deficit of endogenous T3 production (e.g. athyroid patients). Even though in athyroid patients some T3 is still produced by deiodinases in several tissues, previous literature showed an outspoken dissociation between fT4 and fT3 concentrations in this group where even a higher L-T4 dosage did not lead to adequate fT3 concentrations (24). Thus, the adapted peripheral conversion of T4 into T3 in L-T4 users leads to an increased fT4 concentration combined with a not (completely) suppressed TSH, which is reflected in a higher fT4/fT3 ratio".
plus two more ... frontiersin.org/articles/10... Homeostatic Control of the Thyroid–Pituitary Axis: Perspectives for Diagnosis and Treatment Hoermann, Midgley ,Larisch, Dietrich (2015)
frontiersin.org/articles/10... Relational Stability in the Expression of Normality, Variation, and Control of Thyroid Function Hoermann, Midgley , Larisch , Dietrich (2016)
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A defence i've often heard against the GP's accusation of 'overmedication' when we have a low/ supressed TSH is to say "well, my thyroid doesn't need stimulating anymore (or don't exist anymore) so it doesn't matter if i have 0 TSH , what do i need any any TSH for ? ... GP is an idiot"
..............but unfortunately that argument doesn't hold water , it would be nice and simple if it did, but it doesn't.
When we use this argument with a GP, it just confirms their opinion that we don't really understand what we are talking about and that they know best .
TSH has TWO roles in the HPT axis .. Feedforward and Feedback :
once a patient is taking thyroid hormone , GP's are not using TSH to see how much it is stimulating your thyroid ~ (the 'feedforward' role) ~ they DO know that role is now largely redundant.
They are using it as an index/ indicator to show the level of thyroid hormone in your body ~ (the feedback role) ~ and that role is NOT redundant.
Even with no thyroid at all , the TSH level is STILL a reflection of the amount/ effect of thyroid hormone action ON THE HYPOTHALMUS and PITUITARY .
As a result of the level of thyroid hormone action , the Hypothalamus makes TRH, which causes the Pituitary to make TSH...this is the "HPT axis" (Hypothalamus /Pituitary/ Thyroid axis).
The tricky part is to get your GP's to understand that once patients are taking thyroid hormone replacement, the hypothalamus /pituitary's OPINION about their dose can be INCORRECT because the HPT axis is effectively warped by monotherapy (using T4 only) .
When you give T4 only (as a replacement for the T4 AND the little bit of T3 MADE BY THE THYROID GLAND) .. you don't restore that patients ratio's of T4:T3 /TSH to quite the same as their previous healthy settings , and as a result their TSH INDICATOR (opinion) doesn't read quite the same as before.
.... the indicator still works , but the needle has moved~ it says you are doing 75mph (over the speed limit) , when you are only doing 69mph ....just the same as if you change the size of the wheels on a car , the speedo will display the speed incorrectly .
On levo only .... patients end up with relatively higher fT4 /relatively lower fT3 / and relatively lower TSH, than they had when their own thyroid was making a variable amount of ready made T3 .
This part can be proved, and i wrote this post to keep my links to the evidence/ explanation all in one place.
The TSH level is shifted downward a bit on 'levo only' .
Some patients need quite a lot of T4 to get enough T3 from conversion to satisfy ALL their organs and tissues to allow them to feel ok.
..... and this relatively higher level of T4 gives them a lower TSH than they had when they were healthy (because the pituitary is VERY VERY good at converting T4 to T3 ).
... but they haven't actually got as much T3 in the rest of their organs / tissues than they had when they were healthy, due to the loss of that little bit of 'ready made' T3 they used to get from their thyroid. The selfish and greedy pituitary thinks ..."if i've managed to make more than enough T3 from this amount of T4 , then everything else in this body has enough T3 too"
....... but something else in the body might not be as good at converting T4 to T3 as the pituitary is.
So for these patients, a low/ supressed TSH does not necessarily mean they are really overmedicated/ thyrotoxic... these patients are not overmedicated unless they have fT4 / fT3 results and or symptoms which also confirm overmedication.
They may need the higher fT4 (and therefore the lower TSH) to get the same level of T3 .. in order to feel well ~ or they might do better with a bit of added T3 and less T4 to even things up... because there is now some evidence that higher T4 levels can potentially be risky.
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a link to my previous post with list of evidence re. Low vs. Supressed TSH / RISK (it includes a bit about potential risk of high fT4) healthunlocked.com/thyroidu... useful-evidence-that-tsh-between-0.04-0.4-has-no-increased-risk-to-patients-on-levothyroxine-updated-new-study-does-show-small-risk
(I intend to make another post with a list of collected evidence/ explanations showing how T3 lowers TSH relatively MORE than T4 does ~ for use when patients taking T3 need to have low TSH arguments with GP/Endo .. when i've done it i'll add a link to it here )