T3 is a dangerous hormone to take lightly because of its fast powerful action and its short lifetime. In nature, humans with normal function produce 5 times as much T4 as T3 from their thyroids. The T4 is then converted to more T3 in the tissues. We have developed this safeguard of producing a rather inactive prohormone T4 to avoid overproducing T3 – T4/T3 conversion only happens as fast as the cells need it and no faster. There must be a genetically selective reason for this arrangement. When you take T3 alone you are theoretically opening yourself to rapid and for some people unpleasant (and life threatening) effects because you are NOT controlling your T3 uptake like the normal body and thyroid gland does. You don’t have the milder T4 reserve to control T3 production. This does not mean you can’t use T3, but the obligations on you to be more careful than when taking T4 are clear. I do not agree with Dr Lowe about T4 monotherapy – for the majority of people it works well, but by no means all. Sheer facts on the ground refute his position if that is what it is. I’d like as I say slow release T3 to be used because at least you aren’t then getting regular ups and downs in blood levels nad there’s more control. Regarding urinary T4/3: here are a selection of paper references casting doubt on its value:
1) J. F. Finucane Random urine tests in the assessment of thyroid function
Irish Journal of Medical Science December 1976, Volume 145, Issue 1, pp 195-200.
2) Shakespear RA, Burke CW.
Triiodothyronine and thyroxine in urine. I. Measurement and application
J Clin Endocrinol Metab. 1976 Mar;42(3):494-503
3) Wiersinga WM, Fliers E.
Ned Tijdschr Geneeskd. 2007 Dec 22;151(51):2813-2815.
[Determining the thyroid hormones T3 and T4 in the urine: an unreliable test for hypothyroidism]. [Article in Dutch]
4) Changes in renal tri-iodothyronine and thyroxine handling
Edgar J Rolleman1,2, Georg Hennemann1, Hans van Toor1, Christian H H Schoenmakers3, Eric P Krenning1,2 and Marion de Jong2
European Journal of Endocrinology (2000) 142 125–130 ISSN 0804
There are others I can get.