Why is TSH irrelevant on Thyroid Replacement? - Thyroid UK

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Why is TSH irrelevant on Thyroid Replacement?

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Hello Ladies,

Can someone please tell me why TSH is irrelevant when we're on T4 please?

I've searched on here but can't quite find the answer.

Many thanks ๐Ÿ˜ƒ

18 Replies
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I wish I knew!:-)

The only thing I can tell you is that, since starting on thyroid medication 16 years ago (first T4 only for many years, then, in recent years, NDT), the TSH reading has NEVER had anything to do with how I felt, nor with symptom relief.

I had a GP for years who wanted his patients with Hashimoto's disease on TSH suppressive doses of T4 drugs only. He claimed they could not take NDT because that would trigger the autoimmune response. However, he wanted them on enough T4 to suppress their TSH (below 0.2), as that would keep the antibody levels down. Before him, I had another GP who wanted my TSH within range at all cost. All I can say is that, even if T4 drugs only never led to complete symptom relief, I felt so much better once my TSH dropped below 0...so it was obvious that GP was on to something.

That could be related to decreased antibody activity (when my TSH is suppressed, my antibody and anti-thyroglobuline levels stay low as well). Or my suppressed TSH simply means I am finally getting enough T3 into my cells (I am now on NDT).

As I said, I cannot say why the TSH is irrelevant once on thyroid hormone replacement. All I can say is that, for me, a normal TSH (even one low in the range, like 0.5) has never, ever led to complete - or even partial - symptom relief.

Let's suppose your TSH is 0.28 with a reference range of 0.3 - 4.5. A lot of doctors will freak out at that result and insist you are over-medicated and cut your dose, warning about the dire consequences of heart attacks and osteoporosis.

But suppose an enlightened doctor also did a Free T4 and a Free T3 test. They might come out as

Free T4 10.5 (10 - 22)

Free T3 3.3 (3.1 - 6.6)

i.e. both results could be extremely low in range, and the patient probably feels dire. If the dose is lowered the Free T4 and the Free T3 could end up below range.

The really important number is the Free T3. If it is too low then people will suffer hypothyroid symptoms. If it is too high then people will suffer hyperthyroid symptoms. Being over-medicated is really unpleasant, and few people would voluntarily stay over-medicated for very long.

Doctors think TSH is a perfect barometer of how much thyroid hormone the body is producing and also a perfect barometer of how much thyroid hormone the body needs. Patients have discovered over and over again that this isn't necessarily true. Doctors should be looking at the thyroid hormone results directly, not just assuming that the pituitary works perfectly.

MrsBoilie in reply to humanbean

That is a perfect explanation thank you. I know my GP won't want me to stay at 0.10 so I'm trying to arm myself with some understanding.

Many thanks Anna69 and Humanbean. ๐Ÿ‘๐Ÿผ

Clutter in reply to MrsBoilie


Read Treatment Options in thyroiduk.org.uk/tuk/about_... Email louise.roberts@thyroiduk.org.uk if you would like a copy of the Pulse article to show your GP.

I think you're maybe searching for something that isn't there, Mrs Boilie. Many of us will not feel well unless we have a suppressed TSH. More important are the FT3 and FT4 tests which so many GPs / labs refuse to perform if the TSH comes back 'in range'.

A TSH 'in range' for someone who is hypo is taken by many as a gold standard test that all is well...often with the exception of the patient. I was told to go away when my starting dose of levo reduced my TSH to 4.8 and it is only due to the wise and wonderful contributors on this forum and others that I feel much better than I did when under my GP's 'care'.


youtube.com/watch?v=tOb2POQ... ( my experience of being 'treated' by my GP in cartoon form...ish) :D :P

Terrific! ๐Ÿ˜‰

MrsBoilie in reply to Rapunzel

THAT is brilliant! Thank you Rapunzel. ๐Ÿ˜ƒ

I'll give you another explanation:

You have to consider what TSH is, and what it does.

Thyroid Stimulating Hormone. It is a hormone produced by the pituitary. And the pituitary releases it when it senses that thyroid hormone in the blood is low.

In a perfectly working gland, the TSH will stimulate the thyroid to make more hormone. And then the pituitary will reduce production, and the TSH will go down. That's it's job.

But, when you're hypo, for whatever reason, the gland does not work perfectly - sometimes it doesn't work at all! So, we take thyroid hormone by mouth.

The TSH is high when we're diagnosed, because it has been trying to stimulate the thyroid to make more hormone. But, the hormone can't. We then take thyroid hormone orally, and the TSH goes down. Why? Because we don't need it anymore. The gland isn't responded, so we're meeting our needs with hormone from the exterior. The gland is now out of the loup. The pituitary senses that we have enough hormone, so doesn't release TSH. Why would it? We don't need it anymore.

But, doctors have this twisted idea that low TSH automatically indicates a hyper state. It doesn't. It's the high FT3 that indicates a hyper state. Doctors equate low TSH with hyperthyroidism, where long-term high FT3 increases risk for osteoporosis and heart attacks. They cannot seem to understand the difference between being natural hyper, with a gland that produces too much hormone, and therefore suppresses the TSH; and a suppressed TSH when taking thyroid hormone replacement.

If you ask a doctor how a low TSH causes osteoporosis and heart attacks, when all it does is stimulate the thyroid gland, they will not be able to answer your question. Because their assumption is wrong. The TSH has no effect on bones or hearts. It's job is to stimulate the thyroid gland and, basically, that's all it does.

Ipso facto, when you are taking thyroid hormone replacement of any type - but especially something with T3 - the TSH no-longer serves a useful purpose. Unless it goes high, that is, because that would mean you need an increase in dose. But, it really does not matter how low it goes, because it's just not needed. :)

MrsBoilie in reply to greygoose

Greygoose... I have to say.... you are, as always brilliant. Thank you so much. It's a very comprehensive explanation and I actually understand now!

Thanks again ๐Ÿ˜ƒ

greygoose in reply to MrsBoilie

You're welcome. :) And, thank you!

Rapunzel in reply to greygoose

I love it too gg, you are the nuts xx

greygoose in reply to Rapunzel

Thank you, Rapunzel. You're not the first one to tell me I'm nuts! lol xx

leoopard in reply to greygoose

Sadly you're not wrong grey goose. He guidelines from Leeds Teaching Hospitals indicate low TSH should indicate testing of T3 but few doctors do so.

greygoose in reply to leoopard

Ignore my previous remark, just got up, eyes bleery, couldn't read straight! lol Sorry.

I think an analogy of TSH is a throttle pedal in a car.

Moving the throttle pedal sends a signal that results in increasing and reducing the car's speed.

TSH is a signal to the Thyroid gland to increase or decrease its hormone output.

Using TSH to determine if you have a sufficient level of T3 available for cellular absorption (or uptake) is just as bonkers as knowing how fast you are driving only by the position of the throttle pedal.


When you take tablets, whether T4, T3, desiccated thyroid or any combination, and whether you take them once a day or multi-dose, you will see a pattern of T4 and T3 concentrations in the bloodstream which reflects your regime.

This pattern will in turn have its impact on your pituitary its secretion of TSH. (Let us for now ignore the hypothalamus and TRH - that adds to the complexity but doesn't change the argument.)

To trivialise the processes in order to make the issue clear:

A person absorbing 100 micrograms of T4 from a tablet will see a big spike around two hours after ingestion, and then tailing off.

A healthy body might release 100 micrograms of T4 spread across a day. Won't be perfectly even but a lot more even than tablets provide.

The effect of each of these on TSH secretion will be different. Because they are different you cannot take the TSH typical of a healthy person and expect someone on thyroid hormone replacement to have the same TSH level.

Because TSH can become supressed no matter what thyroxine you take.

A prime reason is that control of TSH by T4 therapy is quite different from control of TSH when you are healthy. TSH levels are controlled by T4 AND T3 levels (60% T4 and 40% T3 about). In T4 therapy the FT4/FT3 ratio is higher than in healthy people. Therefore for a given FT4 there will be less FT3. So if you want to get FT3 up to a healthy level, you have to have higher FT4. But this suppresses TSH. This is why TSH levels in treatment should be smaller than in health and around the 0.1 level of less. The TSH range used in measurements is always from healthy people so isn't relevant to those on T4 therapy. But the medical profession haven't twigged this yet.

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