Thyroid UK
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Anyone know of research to explain low TSH not relevant measure in blood tests when taking T3

Hi am just wondering if anyone has links to articles or research that can explain why looking at TSH is not a relevant marker if taking T3?

Been told by my docs to reduce my medication as TSH is 0.08 (fallen from 4.79 in Oct) but FT3 is 4.6 and FT4 is 16.8.

So advised on here I may need to increase not decrease.

Want to be understand this better so I can explain to my docs why I am going against their advice.

Many thanks

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Generally TSH is relevant, it responds to both T4 and T3. However, many patients who require T3 will fall into the following two categories.

1. Their hypothalamic pituitary thyroid axis may be down-regulated, usually due to a previous period of hyperthyroidism or occasionally arising from depression or severe dieting. In this case the TSH is low because of insufficient stimulation from the hypothalamus, not because it is suppressed. These patients will tend to have low fT3, low fT4 with a low or normal TSH. It is important to take the blood about half way between doses if you are taking T3 medication so as to get accurate numbers.

2. Some patients have a peripheral resistance to thyroid hormone. i.e. they are clinically hypothyroid with normal blood test results. They require supra-physiological doses of thyroid hormone to resolve their resisitance to the hormone. This will suppress the TSH.

In both cases, but especially the second one, although it is necessary to have a low TSH it is important to monitor the patient very carefully. You are going beyond normal levels so there is a risk of cardiac harm. Your doctor should also note that if you are left clinically hypothyroid this will also lead to cardiac harm as well as a poor quality of life.

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You could make a start on these two:

Front. Endocrinol., 22 December 2017 | doi.org/10.3389/fendo.2017....

Front. Endocrinol., 20 November 2015 | doi.org/10.3389/fendo.2015....

Put into Google "Frontiers in Thyroid Endocrinology". Then look at left hand column "Most viewed". Second article heading down is one of these. Then look at "most recent" I think this is 3 down. Both articles are downloadable. Head author is R Hoermann both times.

Suppressed TSH need not be an inevitable marker for cardiac problems but needs more scrutiny than if you were taking T4.. It has to be looked at in conjunction with FT3. If FT3 is high above range, then overdosing is certain and should be lowered. TSH below range is inevitable for some people to achieve adequate dosing on T4 but this is not so good for those on T3 only and may indicate too much T3 being taken. But again, only if FT3 is high.

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What if TSH is under range but free T3 is still barely mid range (on T3)?

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This is the best I can come up with as an explanation. In health, TSH is controlled by both T4 and T3 (about equally). Therefore to get an adequate FT3, the TSH to do this is also affected by the FT4. If you have no thyroid and take only T3, then obviously only the FT3 is affecting the TSH. This is different from health, and the body and pituitary will react differently. So I have no problems with TSH being a little low with normal FT3. It's radically different from when FT4 was also present.

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Here is another article showing TSH to be an inferior marker. It also details which things are better indicators of euthyroidism.

tpauk.com/main/wp-content/u...

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That's really interesting - thank you.

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thank you scottbnk for the link.

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You are welcome. I thought the article especially interesting because there are those on this forum who loudly cry that RT3 is harmless and inert, based at least partly on a post by Dr. John Midgely diogenes regarding RT3 having receptors of its own instead of occupying T3 receptors. But the leap was made by others that because RT3 may have its own receptors and not be blocking T3 receptors that it is inert and has no effect on metabolism.

This article favors RT3 having a hypo-metabolic effect and being a better indicator than TSH of cellular status. I believe that it was written by the lead author of the 2 papers diogenes cited in this thread.

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Thank you so much for the informative link.

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