The purpose of thyroid hormone replacement therapy, to treat hypothyroidism, is to replace the low hormone a personís own thyroid gland is no longer able to produce on its own. The two major thyroid hormones involved in this treatment are the T4 and T3, with T4 being on reserve to be converted in the body into the more powerful T3, which is the more metabolically active of the two.
T4 being a reserve hormone is how I describe it in articles but it does have a purpose at the tissue level just as T3 does but only at about 20% compared to T3, according to some medical sources. As a thyroid patient treated for my hypothyroidism with a T4/T3 combination hormone therapy (Armour thyroid brand), I personally want my T3 to be within normal values (not above normal) but I feel less well if mine gets to mid range and especially below mid range.
It concerned me, when I was first blood retested on Armour thyroid brand in early 2004 to monitor my thyroid replacement therapy, seeing a low-ish T4 result, in fact my T3 can be at mid range and T4 will actually be flagged slightly low. I was posting on a MedHelp Thyroid Forum back then and a Board Certified MD named Mark Lupo began answering questions there at one point, so I asked him about T4 being low and he said it was common in Armour patients. Also since then, I've corresponded with and seen the posts of 100s of other Armour patients who report that their T4 stays low-ish on Armour even with T3 at good range for them. It made me wonder at times if Armour doesn't have the correct ratio of T4 to T3 for humans. It apparently does however because Armour went through rigorous scrutiny and approval processes via a number of regulation groups including the USP (United States Pharmacopoeia) and the FDA (Food and Drug Administration). The T4 to T3 ratio of Armour and other desiccated T4/T3 brands is about 4 to 1 (4 times higher in T4). Armour at times over the years has had to recall dose batches that were inconsistent but has happened less frequently over the past several years. The same has happened with synthetic T4 brands, Synthroid having recalls of inconsistent dose batches at times, including a large scale one in 1989 and levothyroxine/levoxyl having a large recall for inconsistent doses in 2002.
Some groups say that desiccated T4/T3 drugs like Armour should have a ratio of more T4 in it than it currently has, that is more like 10 to 1. The manufacturer-Forest Pharmaceuticals however has backing by their medical research reps stating that the current 4 to 1 ratio is the most correct for replacing people with missing hormone in their bodies from hypothyroidism. It is an interesting subject and at times I waiver in my own belief on the subject because I feel at times more T4 should be added to Armour with the Armour dose reduced in doing so. I remember reading the testimonials of other patients successfully taking a regimen like this, years ago and was part of what inspired by belief that more T4 is possibly needed than what is supplied by an Armour dose alone.
Not only has there been controversy on this issue in the US but the UK groups TPA-UK/Thyroid Patient Advocacy and the British Thyroid Association have gone back and forth on this issue. Here's a link that shows some of these debates:> Article Link.
I feel in the US and UK, there is a long way yet to go, in more research and trials needed in these areas for the sake of hypothyroid patients (not always the first consideration) so that there are more definitive answers on these issues. It's similar to how there was and is such issue in regard to TSH blood testing to both diagnose and treat hypothyroidism. There are still top qualified Endocrinologists and MDs who believe anything below a 2.0 TSH is a risky treatment level while others believe a 1.0 should be the target goal for TSH suppression with a thyroid dose to treat hypothyroidism.
The unfortunate thing is that it leaves patients with the need to try different trials of dosing until they find what makes them feel well but is important to have a doctor behind those trials if at all possible (may depend on what country you live in). I'm thankful my doctor is completely behind me in my hypothyroid treatment and her main concern is the same as mine that I stay within normal values on T4 and T3 regardless of the trial of T4 or T3 med I take. My TSH is not accurate, doesn't accurately reflect my thyroid hormones and was determined early on, so thankfully in my case I'm not tied down to TSH even though in most patients it does reflect accurately. These type scenarios and others are what make it difficult for even the most reputable medical research groups to pin areas of thyroid disease treatments down to exacts (unfortunate but true) and is largely due to patient-individuality. There are too many variables that cause patients to have different results from same hypothyroid treatment trials like the TSH issue for example and whether a patient has a goiterous or non-goiterous condition, cold nodules, hot nodules, co-morbid disease processes that change reliability of TSH in these patients etc... I've said many times that medical treatment developments by medical research groups, is a job I would not want to have! It has mountains of difficulty involved with it.
Retesting T4, T3 and TSH in newly treated patients is important.