Investigations:
Investigations were undertaken to rule out TSHoma and the alpha subunits [0.26 IU/L (<3.00)] were normal. Screening for thyroid harmone resistance was done and the SHBG was normal [35 nmol/L (14-71)]. Investigations to rule out assay interference on a different platform (DELFIA) was also undertaken. The free T4 was 22.7 pmol/L (9.0-22.0), free T3 was 7.7 pmol/l (5.0-7.5). Also the serial dilutions showed a linear pattern and PEG showed good recovery. The above tests ruled out assay interference.
Serum sample was sent to Cambridge for further analysis and the total T4 was 259 nmol/L (69.0-141) and Thyroid binding globulin was 14.3ug/ml (14.0-31.0). The discrepancy between the levels of total T4 and free T4 indicated the possibility of a binding protein abnormality. Also the Familial dysalbuminaemic hypethyroxinaemia (FDH) screen was positive.