The measurement of free thyroid hormones and TSH by immunoassay can still be problematic (O’Toole et al. ,1999) Discrepancies may be found between different assays for free thyroxine or there may be inconsistencies between TSH and free thyroxine as measured by routine assays and the clinical picture eg raised free T4 with TSH within the reference range (Gurnell et al., 2011). Before rare causes of this biochemical picture such as TSH secreting pituitary tumours and thyroid hormone or TSH resistance are investigated, analytical interference should be excluded. Such interferences would include antibodies to T4 or T3 (Zouwail et al., 2008), familial dysalbuminaemic hyperthyroxinaemia (FDH) (Cartwright et al., 2009), heterophilic antibodies (Halsall et al., 2009) and ‘macro’ TSH (Halsall et al., 2006).
Investigation of thyroid function tests that are inconsistent or discordant with clinical findings. Please quote local TSH, free T4/free T3 results and methodology on request form.
Analytical methods available:
Anti T4 and T3 antibodies (Birmingham)
Familial dysalbuminaemic hyperthyroxinaemia screen (Birmingham)
Free T4 – by DELFIA two step method, calculated fT4 (using TBG and total T4) and equilibrium dialysis (Cambridge – please use attached referral form).
TSH – method comparison, heterophilic antibody interference screen and dilution studies (Birmingham).
TSH – method comparison, dilution , immunosubtration and Gel Filtration chromatography (Cambridge – please use attached referral form).
None. Take blood (5 mL) using a plain vacutainer or syringe.
Transfer the blood to a plain tube and allow to clot. Separate the serum and store at -20C. Send serum (2 mL) to the SAS laboratory. Comprehensive clinical and laboratory information should accompany each request.
Appropriate reference range information will be provided by SAS laboratory
Centres offering this assay:
1: O’Toole AM et al. Proc ACB Natl Meeting 1999 P37.
2: Gurnell et al. Clin Endocrinol (Oxf). 2011 Jun;74(6):673-8.
3: Zouwail SA et al. Clin Chem. 2008 May;54(5):927-8.
4: Cartwright D et al. Clin Chem.2009 May;55(5):1044-6.
5: Halsall DJ et al. Ann Clin Biochem. 2009 Jul;46(Pt 4):345-6.
6: Halsall DJ et al. Clin Chem. 2006 Oct;52(10):1968-9