As I rather ranted on about at the Thyroid UK conference in 2014, the inability of the various (virtually all tenth rate or less) manufacturers to produce consistent free T4 and free T3 tests, combined with the complacent ignorance of the medical users as to the quality of what tests they actually employ, has led to confusion and poorer diagnostic performance than is acceptable. Thus more consistent TSH has dominated diagnosis. Prof L Thienpont has over the last years been busy collaborating with these manufacturers to try to get some better consistency between products. Free T4 was first in line (products vary by up to 25%). Free T3 was nowhere (manufacturers excuse - few people use the test, hardly any [importantly] in the States, so no pressure to improve from an entirely unacceptable 60% variation source-to-source even though this is quite technically possible). However even though the manufacturers have been dragged into line in theory, they are still dragging their heels over actually doing anything concrete and rationalisation of even Free T4 seems years away. FreeT3 - forget it! In diagnosis, it's like going to the greengrocers in town to get 1 kilo of potatoes, and getting anything between 0.8 and 1.2 kilos. So I hope you can see what an uphill struggle it will be to get the dozy collection of incompetent manufacturers, fast-asleep regulators and complacently ignorant end-users to actually make thyroid function testing acceptably accurate and consistent. As an ex-physical biochemist and molecular biologist, I can tell you my then colleagues would be rolling in the aisles in side-splitting mirth at the deficiencies and the resulting nonsensical situation of using bad test quality and compromising diagnostic efficiency even more than need happen.
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