Total vs Free T4 and T3 tests

[helvella]

A regular question is whether we should test Free T4 and/or T3, or Total T4 and/or T3.

I was reading the current prescribing information for Armour Thyroid (in the USA) and noticed this issue is actually addressed:

Changes in TBg concentration should be taken into consideration in the interpretation of

levothyroxine (T4) and liothyronine (T3) values. In such cases, the unbound (free) hormone

should be measured. Pregnancy, estrogens, and estrogen-containing oral contraceptives increase

TBg concentrations. TBg may also be increased during infectious hepatitis. Decreases in TBg

concentrations are observed in nephrosis, acromegaly, and after androgen or corticosteroid

therapy. Familial hyper- or hypo-thyroxine-binding-globulinemias have been described. The

incidence of TBg deficiency approximates 1 in 9000. The binding of levothyroxine by TBPA is

inhibited by salicylates.

rxabbvie.com/pdf/armour_thy...

TBg - Thyroid binding globulin

TBPA - Thyroid binding pre-albumin - more often called Transthyretin

To me, it seems madness to use the Total assays, then shift to the Free tests because one or other of a variety of seemingly disparate conditions subsists! If nothing else, it renders any comparison with previous Total results ridiculous.

[tattybogle]

"pregnancy , estrogens , oral contraception"..... lol ...should read "most adult female members of the species"

[helvella Administrator in reply to tattybogle]

diogenes used to say pretty much that.

In the UK, it looks like the last remnant of total testing are the labs which insist on Total T3. Seemingly on the grounds of Free T3 being "too variable" or something like that.

[tattybogle in reply to helvella]

(Spock) ......."that's illogical captain"