The thyroid gland makes two main hormones – thyroxine (T4) and tri-iodothyronine (T3). T4 is produced in significantly greater quantities (a ratio of 17:1) than T3 whilst T3 is the more active hormone of the two. In fact T4 functions as a pro or storage hormone, it does not enter the cells and is biologically inert. In circulation, most T4 is bound to specific transport proteins.
These transport proteins, produced by the liver – thyroid binding globulin (TBG), transthyretin and albumin – carry T4 and T3 to the tissues, where they are cut from their protein-carriers to become free T4 and free T3 and bind to thyroid hormone receptors (THRs) and thus exert their metabolic effect. Therefore it is critical that your doctor tests both free T4 and T3, as it is the unbound hormones that are able to do their job.
Unfortunately many GPs will only test for T4, total or possibly free and, like me if your body cannot convert T4 to T3 then a true picture of your thyroid health will not be obtained. You may have a normal blood level of T4, total or free, but it can’t be utilized by your body. The optimum levels for these tests should be roughly mid-range, however if either or both are high/low or a combination of the two there generally is an issue.
Remember we are looking for optimum health – not ‘within range’ which in mainstream medical terms can often translate to near enough is good enough despite clinical symptoms!