CLARIFY THE ROLE OF TSH

[fiftyone]

Hi, is TSH sensitive to only T4 in the bloodstream or also to T3?

[Marz]

Good question

[SlowDragon Administrator]

Both.

TSH generally more sensitive to FT3, as many patients find as soon as start any T3, TSH becomes suppressed even if FT3 and/or FT4 are still low

Replacement hormones taken via stomach, once a day, or even in divided dose(s) does not replicate a normally functioning thyroid where output responds to constantly fluctuating demand

[fiftyone in reply to SlowDragon]

So if my TSH is greatly suppressed, 0.002, but my T3 is within range and my T4 is high (22/24) what is my TSH responding to? Looks to me like it's only responding to T4. Puzzled. I take thyroxine.

[humanbean]

You might find this thread of interest :

healthunlocked.com/thyroidu...

[jimh111]

The pituitary receptors respond to T3 to reduce TSH secretion. The pituitary is able to convert T4 to T3. So it responds to both. If fT3 and fT4 are average you will most likely have an average TSH. If one is average and one is high your TSH will be low. Your fT4 is high so your TSH is suppressed. It is the combined effects of T3 + T4 that affects TSH.

In the blood fT3 is about 4x - 5x as effective in supressing TSH as fT4. Taken as tablets liothyronine is about 3x as effective as levothyroxine. So, 40 mcg liothyronine will have the same effect on TSH as 120 mcg levothyroxine. How much TSH is suppressed by T3 or T4 depends upon how much is taken. TSH is often suppressed in patients taking T3 because they take a lot of T3, or a lot of T3 and T4. Some patients have a low TSH with normal fT3, fT4 whether they take T3 or T4, these patients seem to have a minor pituitary problem.

[fiftyone in reply to jimh111]

I thought T4 was converted to T3 in the liver? And I was under the impression that the TSH was not a hormone which circulated round the body but just gave 'messages' to the thyroid.

[diogenes Remembering in reply to fiftyone]

T4-T3 conversion occurs in many cells in the body. TSH has many subtly different forms, each of which have different effects. One form stimulates the thyroid, another may affect bone metabolism, and others again body heat production. TSH has many major and minor influences throughout the body.

[helvella Administrator in reply to fiftyone]

But a TSH blood test would not be possible if the TSH didn't get carried round the body by the blood!

[jimh111 in reply to helvella]

Which is a relief, I wouldn't want a needle stuck in my neck!

[jimh111 in reply to fiftyone]

The liver does some T4 to T3 conversion, most conversion is done in other tissues.

[SlowDragon Administrator]

Here are few links

restartmed.com/low-tsh/

tiredthyroid.com/blog/2014/...

jeffreydachmd.com/2016/10/a...

thyroiduk.org.uk/tuk/thyroi...

Dr Toft, past president of the British Thyroid Association and leading endocrinologist, states in Pulse Magazine,

"The appropriate dose of levothyroxine is that which restores euthyroidism and serum TSH to the lower part of the reference range - 0.2-0.5mU/l.

In this case, free thyroxine is likely to be in the upper part of its reference range or even slightly elevated – 18-22pmol/l.

Most patients will feel well in that circumstance. But some need a higher dose of levothyroxine to suppress serum TSH and then the serum-free T4 concentration will be elevated at around 24-28pmol/l.

This 'exogenous subclinical hyperthyroidism' is not dangerous as long as serum T3 is unequivocally normal – that is, serum total around T3 1.7nmol/l (reference range 1.0-2.2nmol/l)."

You can obtain a copy of the articles from Thyroid UK email print it and highlight question 6 to show your doctor

 please email Dionne at

[e-mail]

Professor Toft recent article saying, T3 may be necessary for many otherwise we need high above range FT4 and suppressed TSH in order to have high enough FT3

rcpe.ac.uk/sites/default/fi...

[SlowDragon Administrator]

Approx 20% conversion takes place in the gut. Hence the reason so many people with gluten or lactose intolerance , due to impaired gut function, also need addition of small dose of T3