I must repeatedly declare that notorious RIA and/or ELISA is not quantitative at all. Instead, I have recently developed quantitative PDMD method, which depends on reliable quantitative HPLC-photometric method for determining PTH-aminoacids derived from the relaiable Edman degradation method (please see file; HepG2 fucoidan).
By the way, I have searched TSH/Thyroid stimulating hormone and TRH/TSH releasing hormone in my liver and serum specimens by PDMD method in vain.
On the other hand, Met-enkephalin seems to be present at μM order, since Human Serum Biotinidase/Enkephalin amino-exo-peptidase shows Km of μM order at the homo-tetramer structure (please see file; The Fascio effect).