Is there Relationship between TSH, FT3, and FT4... - Thyroid UK

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Is there Relationship between TSH, FT3, and FT4? How could I Improve my FT4 and FT3 levels?


I would appreciated member's comments on the following questions:

(1) is there an accepted optimum relationship or ratio that we should seek between TSH, FT3, and FT4 levels? For example, should FT3 or FT4 be a certain % of TSH levels of FT3 be a % of FT4 levels.

(2) After TSH levels, which is more important, a good level of FT3 or FT4?

(3) How could I improve my FT3 or FT4 to achieve better health?

(4) Are FT3 and FT4 the main levels after TSH that we should focus on for better health? I suffer from fatigue, feelings of being tired even after 8 hours of sleep, and low libido. (My Vitamin D, B12, Iron, Ferritin, Folic etc. levels are good and well within range). My testosterone are within range but mildly low at 12.51 mnol/L against range of 9.90-27.80. My TSH is good at 0.482 micU/mL against range of 0.27-4.2.

(5) How could I convert pmol/L to ng/dL. I have changed testing laboratory and the new lab publishes FT4 results in ng/dL whereas my last lab used units of pmol/L. Similarly, my Vitamin D results used to be published in units of nmol/L but the new lab uses units of ng/mL. How can I convert them so I can compare new results with old (i.e. like for like).

Thank you.

23 Replies


My opinion, and maybe not explained as well as others might do

1) I can't see how there is. We are all individuals and need our own personal levels where we feel best. What's right for me isn't necessarily right for you or anyone else.

2) TSH isn't the most important. It's not a thyroid hormone, it's a signal from the pituitary to tell the thyroid to produce hormone. FT3 is the most important result as T3 is the active hormone that every cell in our bodies need. If we feel unwell and symptomatic, a low FT3 will show that but TSH wont.

3) Optimal levels of thyroid hormone and optimal nutrient levels, addressing any gut/absorption issues, addressing Hashi's if present.

4) Once diagnosed, TSH is irrelevant. As mentioned above, it's FT3 that's important.

My Vitamin D, B12, Iron, Ferritin, Folic etc. levels are good and well within range

But are they optimal, within range isn't enough. Optimal levels are

Vit D - 100-150nmol/L

B12 - top of range

Folate - at least half way through range

Ferritin - half way through range.

My TSH is good at 0.482 micU/mL against range of 0.27-4.2.

Yes, that's good, most hypo patients feel best when TSH is 1 or below, but what are your FT4 and FT3 levels?

5) How could I convert pmol/L to ng/dL - as far as FT4 s concerned, don't bother converting. Just work out the percentage through the range, eg

FT4: 16 (12-22) = 40% through range

Vit D - to convert nmol/L to ng/ml you divide by 2.5 and to convert ng/ml to nmol/L you multiply by 2.5, eg

150nmol/L divided by 2.5 = 60ng/ml

40ng/ml x 2.5 = 100nmol/L

Or you can just be lazy and go to, which has converters for most hormones, vitamins etc. For example,

Thank you Angel. Have noted websites and am using them. Very useful.

Adam10 in reply to SeasideSusie

Many thanks for your detailed reply SeasideSusie.

My Endo did not test FT3. He tested only TSH and FT4.

My FT3 result from a year ago was 4.09 (range 2.8-6.8) so 60% of range (is that how you calculate it).

My pituitary gland is not secreting enough hormone. My FSH and UH results are low.

I suffer from Hashimoto's.

My Endo said don't test antibodies which last year were TG 131 (range 0-115) down from 501 in 2014, and TPO 38 (range 0-34) down from 175, I think due to gluten free diet.

My other test results last year were:

1. Vitamin D, 140 nmol/l, so within optimal 100-150.

2. B12 882 (range 211-945) so 93%.

3. Folate (not tested)

4. Iron 26 (5-28) so high.

5. Folic Acid 26 (7-45) just above halfway.

6. Ferritin 92 (30-400) only 24% of the range. Could this cause my fatigue. What could I do to boost my Ferritin?

Grateful for any further thoughts. Thank you.

Your free t3 was about 30% of range - not 60%. The range is 4 long (6.8 minus 2.8). If your level was 3.8, it would be 1 whole number through the range - 3.8 minus 2.8 - or 25% (1/4 = 25%). It's 1.22 numbers through the range (1.22/4 = 30.5%). It's just arithmetic! Back to primary school for you ;-)

Another quick calc is here: . Not in English but that doesn't matter. It goes Value, Bottom of range, Top of range, click big button, look at bottom of screen for result.

Thanks. Just used the calculator. I see what you mean. My FT3 is 32% of range and my FT4 is 40%.

What does this mean and how do I increase my FT3 and FT4? My TSH is already low at 0.48. Do I increase my T4 further or look at adding T3 (never done this).

SeasideSusieAdministrator in reply to Adam10


My Endo did not test FT3. He tested only TSH and FT4.

Well, that's typical I'm afraid. They're so poorly educated about hypothyroidism they don't understand just how important FT3 is when hypothyroidism is being treated.

My FT3 result from a year ago was 4.09 (range 2.8-6.8) so 60% of range (is that how you calculate it).

No, it's 32.25% through the range. Half way through the range would be 4.8 (2.8 + 6.8 = 9.6 / 2 = 4.8) so you can see from that you're not 60% through range.

I use an equation to work it out but the link to the calculator that AOTN has given gives the same result.

Not sure how you're working out your percentages but I get

2. B12 882 (range 211-945) so 93%. - 91%

5. Folic Acid 26 (7-45) just above halfway. - 50%

6. Ferritin 92 (30-400) only 24% of the range - 16.75%

Could this cause my fatigue

Quite likely. Have a look at these articles:

What could I do to boost my Ferritin?

Eat liver every week, maximum 200g due to it's high Vit A content. If you don't like liver, hide it in casseroles, cottage pie, curry, etc. Or eat liver pate. Black pudding also good. Other iron rich foods

But last year's results aren't necessarily relevant now, they could be better now or could be worse.

Endo is correct when he says not to test antibodies, there's no need, you know you have Hashi's. Just stick to a gluten free diet and supplement with selenium l-selenomethionine 200mcg daily and you'll be doing your best to help reduce the antibodies. They're going to fluctuate anyway so no point in testing, and they'll be there until your thyroid is destroyed.

1) There is not exact correlation between levels of TSH and levels of FT4 and FT3. In a euthyroid person (with a normal thyroid) there will be a ratio range - not a specific percentage but a percentage range - between FT4 and FT3. But, ratios go out the window when you're hypo because hypos do not always have the capacity for efficient conversion.

3) If your thyroid cannot produce enough hormone to keep you well, the only way you can raise your Free levels is by taking thyroid hormone replacement. And taking enough to raise your levels and keep them optimal. Nutrition etc. helps, but your cannot replace a hormone with a vitamin, or anything else.


Adam10 in reply to greygoose

Thank you greygoose.

I have increased my 100 mcg Levo from daily to 100 mcg five days and 125 mcg two days per week. This has lowered my TSH from 1.03 to about 0.48.

But my fatigue, irritability, and low libido remains.

I eat a healthy diet and do walking exercise almost daily.

My past FT3 was 4.09 (range 2.8-6.8) so about 60% and my FT4 was 16.09 (12/22) so about 73%. But I am still fatigued and lethargic.

greygoose in reply to Adam10

I'm not very good at maths, but I really don't think 4.09 is 60% in a range of 2.8-6.8. 50% is 4.95. So, you're not a very good converter and your FT3 is too low. A lot of hypos need it up the top of the range to feel well. The TSH is not relevant to the way you feel.

Adam10 in reply to greygoose

Thanks Greygoose. I shall stop thinking about TSH linked to how I feel. I’ve been using that wrongly for sometime. How do I improve my conversion of FT3 and increase it from 4.09 to nearer 6.8 please?

greygoose in reply to Adam10

There are many, many reasons for poor conversion. First problem is finding the reason - which isn't always possible - second problem is that reversing it is not always possible.

For example, do you have Hashi's? Have you had your antibodies tested? Hashi's people often have poor conversion and there's nothing you can do about it. Have you had your vit D, vit B12, folate, ferritin, zinc, selenium, and possibly others tested? Low nutrients are sometimes the cause of poor conversion. How about your cortisol? Are you on a low-calorie diet? Have you ever been? Do you get enough carbs? The list goes on.

So, while your trying to find the solution, if you even can, people find it a good idea to procure some T3 and add that to a reduced dose of levo. Difficult to get it prescribed these days, so most people buy their own, but worth trying to get it out of the doctor, first.

Having said all that, I've just has another look at your results… How much levo where you taking when that test was done? FT4 was 16.09 (12/22)? No way is that 75%. I think you were working your percentages out backwards. :) 50% is 17, so you were undermedicated when that was tested. Your FT3 could be better now, if you've increased the dose. So, before launching into the search for the holy grail (T3), get retested and see what your levels are now. :)

Adam10 in reply to greygoose

I was taking 100 mcg daily now I’m taking 100 mcg five days and 125 mcg 2 days per week which reduced my TSH from 1.0 to 0.48 but FT3 not recently tested. In the past FT3 was highest at 5.76 but 4 has been typical for 2017.

Adam10 in reply to greygoose

My endo did not mention the conversion of FT3 and did not include iFT3 test in my recent blood tests. Do they not recognise the conversion issue? Is it only Thyroid UK etc and HealthUnlocked that consider it.

roxanaleah in reply to Adam10

According to

(found under the heading:

l-Thyroxine Monotherapy Fails to Restore All Markers of Hypothyroidism)

"higher serum T4 levels will impair systemic T3 production via downregulation of a deiodinase pathway (9)."

Though it might be counterintuitive and some people might find it threatening to their conceptions, it is possible that overmedication is causing HYPOthyroid symptoms. Yes, I wrote that properly.

Also read Dr. Alan Christianson's e-book: "Healing Hashimoto's: A Savvy Patient's Guide"

Healing Hashimoto's: A Savvy Patient's Guide - Dr. Alan Christianson

PDFDr. Alan Christianson › uploads › 2014/10

He explains this phenomenon very well.

I have improved my quality of life, dramatically, by reducing my dosage.

Adam10 in reply to roxanaleah

Very interesting and hope inspiring RoxanaLeah. Thank you. I will buy the book and research.

Yes it does sound threatening when we have all thought that being under-medicated causes the fatigue when it could be the opposite.

Does it mean that levothyroxine (T4) alone cannot satisfy the under production of T4 by our damaged or missing (removed) thyroids ie do we need to consider other medications in combination with T4.

roxanaleah in reply to Adam10

The book is free for viewing online.

I believe the pertinent information is on pages 27-30.

The thing is, I've read, time and time, again, about a return of lethargy and "hypo" symptoms from people whose TSH levels indicate suppression.

Some suggest that TSH results are meaningless. I do not share that opinion.

Besides our symptoms, which I am convinced are not understood properly, the results of TSH, Ft3 and Ft4 tests are the only data we have upon which to base our treatment plans. Chucking 1/3 of that data because of fear or other feelings just does not seem wise to me. Some place great value in the TSH in terms of revealing underactivity and yet disregard it, entirely, after that.

Dr. Christianson (a subject matter expert) has concluded, from empirical evidence, that suppression of the TSH is unhelpful. Since its role in conversion is not understood, and because functioning thyroid glands result in a measurable level of TSH, I refuse to ignore it. And, anyway, I was unable to achieve a sense of well-being with a suppressed TSH level.

There was one symptom you identified that prompted me to respond: irritability. In my own experience, an 'on edge', even angry, mood is associated with overmedication. Also, sweating or feeling warmer than is usual, even slightly.

I hope you find value in the links I included!

Adam10 in reply to roxanaleah

Thanks roxanaleah, really fascinating. I’m hoping what you say could be my solution. I will try anything to get well and energetic again. My TSH has been suppressed for years but I still feel fatigued and irritable.

The book sounds full of hope.

My irritability and feelings of being ‘on edge’ are damaging to my work and married life. I am constantly impatient with work colleagues, friends, and my wife.

I regularly have night sweats still and I feel hot when others feel cold.

The fatigue is present and a life burden. I want to sleep at 9am after having slept for 8 hours overnight.

I am praying that Dr Christianson’s book can make a difference.

Thank you so much for your information and responses.

SeasideSusieAdministrator in reply to Adam10


My past FT3 was 4.09 (range 2.8-6.8) so about 60% and my FT4 was 16.09 (12/22) so about 73%

As mentioned above FT3 was 32.25% through range, and FT4 was 40.9% through range. You were undermedicated with those results. But as you have increased your Levo since those tests were done and now have a TSH of 0.48, you need to know your new FT4 and FT3.

Adam10 in reply to SeasideSusie

Thanks seasidesusie. Will go back for FT3 test.

Will manage my requests more assertively as my new endo is unknowledgeable or unaware of the importance of what you and greygoose have mentioned.

I wonder if my endo ignored testing of FT3 because he thinks low conversion rates of FT4 to FT3 can’t be rectified or are too difficult to rectify.

My endo didn’t raise ferritin or the other issues but worst of all he relied on results within range being satisfactory instead of requiring optimal levels as you have clarified.

For others reading this, it is yet again reason to control our treatment rather than leaving it to the doctor or even an endo.

SeasideSusieAdministrator in reply to Adam10


I wonder if my endo ignored testing of FT3 because he thinks low conversion rates of FT4 to FT3 can’t be rectified or are too difficult to rectify.

Most endos are diabetes specialists so don't know much about thyroid anyway. They don't understand the importance of it, they're not taught it. T3 is the active hormone that every cell in our bodies need. I believe it should be done as part and parcel of every thyroid function test.

Dr Toft, leading endocrinologist and past president of the British Thyroid Association, stated in Pulse magazine (the magazine for doctors)

"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 article by emailing Dionne at print it and highlight question 6 to show your doctor.

Dr Toft has now written another article which states that T3 may be helpful for many patients

My endo didn’t raise ferritin or the other issues but worst of all he relied on results within range being satisfactory instead of requiring optimal levels as you have clarified.

This is something else that doctors don't understand. They are taught that anywhere within the range is fine so it can't be causing the patient a problem. Take your range for ferritin - 30-400. As far as they're concerned, anyone with a level of 30 hasn't got a problem, neither has someone with a level of 400. But the reality is that someone with a level of 29 or 401 would have a problem. And think how different Patient X would feel with a level of 30 to how he would feel with a level of 400. There is a level at which we are all at our own optimal level, and with your range that is somewhere within the 370 point difference between the top and bottom of the range. Numbers aren't everything, symptoms are, they just don't get that, doctors are taught to look at numbers only and ignore symptoms.

One other thing to take into consideration, you have Hashi's, you can experience symptoms of hypo and hyper, that's the nature of Hashi's. If you haven't already done some research, here's a few articles to get you going:

And as far as numbers are concerned - TSH and FT4 - there really is no one size fits all, it's all so very individual. I have been keeping a record of my results for the last 20+ years (diagnosed 43 years ago). My TSH has always been suppressed or just scraped into bottom of range, regardless of where my FT4 is or how much thyroid meds I take. Just as an example

TSH: 0.5(0.0-6.0) FT4:26.6(10.3-25.7) on 150mcg Levo - functioning

TSH: <0.2 0.2-4.0) FT4: 25.8(11.8-24.6) on 150mcg Levo - functioning

TSH: 0.03(0.27-4.20) FT4:28(11.8-24.6) FT3:5.9(2.8-7.1) on 125mcg Levo

and here my GP doesn't understand the results. Eventually got referred to an endo who wanted my TSH in range so he reduced Levo to 100mcg and this is the result of that

TSH: 0.4(0.27-4.20) FT4:15.6(11.8-24.6) FT3: 2.8(2.8-7.1)

so the endo had achieved what he wanted to and was very happy, I ended up extremely ill and needed looking after for 2 years. Fortunately my GP could see what the endo had done to me and disagreed with him, she increased my Levo but a lot of damage had been done.

Now, after learning what's important - optimal nutrient levels, etc, and adding T3 myself to my prescribed Levo, my FT4 and FT3 are within the range and where they need to be for me and my TSH is still suppressed, it would appear that's not going to change.

So, it's all very individual. We can all tell you our experiences, we can read all the research we like, but at the end of the day we have to find what works for us.


As you have Hashimoto's and your vitamin levels are now pretty good you need to retest thyroid INCLUDING FT3

For full Thyroid evaluation you need TSH, FT4, FT3 plus TPO and TG thyroid antibodies and also very important to test vitamin D, folate, ferritin and B12

Private tests are available. Thousands on here forced to do this as NHS often refuses to test FT3 or antibodies or vitamins

Medichecks Thyroid plus ultra vitamin or Blue Horizon Thyroid plus eleven are the most popular choice. DIY finger prick test or option to pay extra for private blood draw. Both companies often have money off offers.

All thyroid tests should ideally be done as early as possible in morning and fasting. When on Levothyroxine, take last dose 24 hours prior to test, and take next dose straight after test. This gives highest TSH, lowest FT4 and most consistent results. (Patient to patient tip, GP will be unaware)

Also look at DIO2 gene test. This is common gene variation


Also as you have Hashimoto's have you tried, or are you currently on, strictly gluten free diet?

Often essential, before considering adding T3.

According to Izabella Wentz the Thyroid Pharmacist approx 5% with Hashimoto's are coeliac, but over 80% find gluten free diet helps, sometimes significantly. Either due to direct gluten intolerance (no test available) or due to leaky gut and gluten causing molecular mimicry (see Amy Myers link)

Changing to a strictly gluten free diet may help reduce symptoms, help gut heal and slowly lower TPO antibodies

Ideally ask GP for coeliac blood test first

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