Testing of T3

January 28, 2012

Dr. P suggested I got T4/T3 checked at Genova and the 24 hour urine was

recomended. Genova also recomended that one. So with shock (or maybe I should

not be shocked) read this response on the talk health forum when someone asked

about why the NHS dont check T3. Here is the link but I will also quote

6076

quote

Urine tests of thyroid hormone status are scientifically invalid and they

produce misleading clinical information. They should not be used. There is not a

health service in the developed world that advocates urine testing of thyroid

hormones and there is not an internationally accepted scientific publication

that provides authoritative evidence in favour of their use.

I will try to explain the above strong statement.

Everyone makes T4 as a storage form of thyroid hormone. We metabolise it in one

of two ways - both of which remove one of the four iodine atoms. We either

produce active T3 or biologically inactive RT3. At any point in time there is a

balance between these two forms of metabolism. Whenever, anyone has a chronic or

acute illness, has surgery or feels generally unwell it makes physiological

sense to slow down metabolism and this is achieved in part by a switch in the

balance of metabolism of T4 so that relatively less T3 and relatively more RT3

are produced. In severe illness these changes can be detected in serum T3

measurement but usually the effect is more minor. However, urine concentrates

the excreted forms of thyroid hormone and so the effect of less T3 is more

readily seen in urine, even in people with a modest degree of illness. The

outcome is that a very high number of unwell subjects will have a 'low' T3 in

urine when compared to a healthy population reference range. This is a normal

and necessary response to illness it does not mean that they have

hypothyroidism. As they get better so their urine T3 results will return to

normal. People with 'low' T3 in urine do not require replacement T3.

The only biochemical tests that have value in the diagnosis of hypothyroidism

are serum TSH and T4 (usually measured as FT4). Of these two the TSH is the most

useful.

Neither T3 nor RT3 measurement add anything to the diagnosis of hypothyroidism -

for the reason given in the previous paragraph.

unquote

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