Useful study on effect of central hypothyroidism on T4 and T3 production

[diogenes]

This may interest those who have been diagnosed with cenral hypothyroidism

doi:10.1016/j.endonu.2010.09.006

Serum free triiodothyronine (T3) to free thyroxine (T4) ratio in treated central hypothyroidism compared with primary hypothyroidism and euthyroidism

Gemma Sesmilo a,∗ , Olga Simó c , Lucía Choque b , Roser Casamitjana d , Manel Puig-Domingo b , Irene Halperinb

Abstract The standard treatment of hypothyroidism (central and primary) consists of thyroxine (T4) administration alone. However, the normal thyroid gland produces a small proportion of triiodothyronine (T3) directly into the circulation.

Aim: We aimed to study the free T3 to free T4 ratio in treated central hypothyroidism compared with euthyroidism and treated primary hypothyroidism.

Methods: Eighty-three subjects were included in this cross-sectional study: 36 with central hypothyroidism, 20 with primary hypothyroidism and 27 healthy controls. A clinical history and a physical examination, including height and weight measurement, were performed and body mass index (BMI) was calculated. Fasting blood was drawn to measure T3, T4, free T3, free T4 and TSH.

Results: The free T3 to free T4 ratio was lower in treated central hypothyroidism than in euthyroidism but was similar to treated primary hypothyroidism. Free T4 was higher in treated central and primary hypothyroidism than in euthyroidism. Age, sex and BMI did not affect the free T3 to free T4 ratio.

Conclusions: Treated patients with central hypothyroidism had a lower free T3 to free T4 ratio, similar free T3 levels and higher free T4 concentrations than euthyroid controls, whereas all these parameters were similar in central and primary hypothyroid patients treated with T4. The question of whether these findings translate into adequate tissue concentrations of free thyroid hormones in all tissues remains to be answered. Further studies should aim to determine whether clinical outcomes could be improved by a treatment achieving more physiological plasma concentrations.

[LindaC]

Thank you once more, diogenes. I would just love to know WHICH kind of hypothyroidism I had/have... clearly medicine - as practiced - is not much geared up for such 'trivia'.

So wish you et al were advisers to mainstream Endo departments!? 🦕👽

[Musicmonkey]

Thank you diogenes

[humanbean]

However, the normal thyroid gland produces a small proportion of triiodothyronine (T3) directly into the circulation.

The thyroid produces approx 20% of the body's supply of T3, the rest coming from conversion in other organs and tissues.

I wouldn't consider 20% to be a small proportion of all the T3 available, simply because it makes so much difference to how well people feel.

A few questions...

When I've read that 20% of T3 comes from the thyroid directly, is that referring to Total T3 or Free T3?

I know this next question isn't related to thyroid, but I thought I'd ask...

In connection with cortisol...

I know the terminology for cortisol doesn't refer to Total cortisol and Free cortisol, but instead it is usually referred to as Bound and Unbound, but as far as I know the idea is the same as Total and Free for T4 and T3. How is "Total" Cortisol found in blood converted into "Free" Cortisol in saliva? And why can some people have very good levels of cortisol in blood but virtually none in saliva?

[diogenes Remembering in reply to humanbean]

The 20% T3 coming from the thyroid direct is in total T3 terms. Since T3 is more weakly bound (by a factor of 8-10) then if one unit of total T4 would give x amount of FT4, one unit of T3 will givel 8-10x amount of FT3. Also the estimation of 20% of T3 from the thyroid is only an average and not accurate at that (see Tania Smith's demolition in thyroid patients.ca)

[humanbean in reply to diogenes]

Thank you.

[TSH110 in reply to humanbean]

It’s a really good read and whenever I see that figure quoted as a fact set in stone my blood boils, it’s dynamic 🙄 :

thyroidpatients.ca/2019/05/...

[humanbean in reply to TSH110]

Thank you.

[userotc]

I note the fT3/fT4 ratios are >2 in the report? I understand they should be >0.33 (my latest = 0.37) but mid-normal range for my labs would be ~0.3 with fT3 and fT4 resp = 3.1-6.8 pmol/l and 12.0-22.0 pmol/l. Please clarify.

[tattybogle in reply to userotc]

'Ratio' has probably been calculated 'the other way round', (you will see both methods used by different people ) :

Example of someone with fT4.... 13, and fT3 ... 4

T3:T4 ratio (T3 divided by T4) eg 4 divided by 13 = 0.30

When done this way, a higher number = a 'better' result ('better' level of T3)

T4:T3 ratio (T4 divided by T3) eg 13 divided by 4 = 3.25

When done this way, a lower number = a 'better' result

[userotc in reply to tattybogle]

Thanks but it does seem to be fT3/fT4 ratios that are presented. Table 1 also lists individual fT3 and fT4 figs eg euthyroid 3.02 and 1.16 so ratio fT3/fT4 = 2.6. But units for fT3, 4 differ?

[tattybogle in reply to userotc]

my writing of (T3:T4 ) and (T4:T3) was just to be clear what i meant .. i've seen it written in various papers in either way , and the way it's written doesn't always conform to way they did it .

Information on Ratio's are very tricky to interpret, unless you have the full details of the data used in the study paper, and especially if different units / or same units but different lab ranges, are used . it's hard to know what calculations they actually did .

Even when you have the same units for T3 and T4 i'm sure lab ranges should be taken into account... some people on forums seem to think you can work out the ratio by using just the T4/ T3 results with no reference to the lab range used for those results.. but i'm sure that's not going to to be accurate .

I have some reservations about assessing conversion definitively by using either of these methods (that simply divide the fT4 and fT3 results ) because they take no account of the wildly differing lab ranges that could have been used for fT4 tests . (and to a lesser extent, fT3 tests)

eg . A blood sample with a 'high' fT4 level done on a platform using a [12-22] range . would be '22'.

but the same blood sample with the same 'high' fT4 level done on a platform using a [7.9-14] range would be '14'

these large differences in fT4 numbers have a significant impact on the results of ratio calculation when done this way.