Speaking of model organisms including mice, Xenopus and zebrafish, would a total T4/T3 test be a better indicator for hypothyroidism over free T4/T3? Also, out of T4 and T3, which would be a better candidate to test for the same? Any advice would be appreciated. Thank you!
About 95% of thyroid hormone is bound to plasma proteins - thyroid binding globulin (TBG), constituting 'total thyroid hormone'. The remainder, approximately 5%, is 'free' in the circulation and therefore metabolically active. It's worth noting that T4 is a pro-hormone, which is converted into T3 by 5'-iodinase. T3 has a higher metabolic activity than T4, but has a shorter half-life. Taking this into account, I would say free T4 (FT4) is a good assessment of thyroid hormone levels. It would also be worth testing FT3, given that there can be issues in the conversion of T4 to T3. It may also be worth testing for thyroid stimulating hormone (TSH), which if elevated, along with a depressed FT4 result, is indicative of hypothyroidism. Here's some more information: http://labtestsonline.org.uk/understanding/analytes/tsh/tab/test/. Hope that's helpful!
Thyroid network is regulated by a redundant system and total T4 pool is sensitive to the thyroid secretion, the binding with plasma proteins and the effects of peripheral metabolism of iodothyronines. If the "free hormone" hypothesis holds true, fT4 is in equilibrium with bound T4 and cellular iodothyronines network. So far, subtle changes of thyroid homeostasis may be better recorded by fT4 than T4
Dear Kunal, The better first line test for evaluating hypothyroidism, is TSH. Then, if you want to know if it is subclinical (normal peripheral hormones) or clinical ( low peripheral hormones), you can measure hormones peripheral ( unless there are problems with binding proteins , is the sato measure free T4 or total T4. T3 is not useful for hypothyroidism since it must be very marked low at the final stages to mark hypothiroidism. As for TSH you must take into consideration that some commercial kits measure specific forms of species, for example you cannot measur canine TSH with kit for human TSH. Regarding to the species you mention, I do not know if there are commercial kits ( guess not) , so, you would have thought in "in house" methods specific for those species. For total or free T4, there is no the problem with specificity of species and you can measure them with kits for humans, but taking into account that the reference values may not be the same for humans, and if you don't know them, you can not diagnose hypothyroidism anyway. Best regards. Liliana
I canj not add anything to the above which I agree with.In brief best to measure FT4 and TSH for hypothyroidism and FT4,FT3 and TSH for hyperthyroidism
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