Sethsiri Wijeratne1, Sara Jenks2 and Hala Alsafadi2 “I am taking my thyroxine regularly doctor!” Sethsiri Wijeratne1, Sara Jenks2 and Hala Alsafadi2 Warwick Hospital1, Warwick, UK, and George Elliot Hospital2, Nuneaton, UK. Introduction Hypothyroidism frequently affects women of childbearing age. Inadequately treated hypothyoidism in pregnancy is known to have significant maternal and foetal complications. We report the case of a patient with under-treated hypothyroidism during pregnancy because of artefactual error in immunoassay thyroid function measurement. Case Report A 33 years old female 8 weeks into her 4th pregnancy was referred to the obstetric medical clinic for monitoring of her hypothyroidism Her medications included 100 micrograms levothyroxine daily. Past obstetric history included maternal anaemia and a post partum haemorrhage (PPH), both of which have been linked with maternal hypothyroidism. On reviewing in the clinic she complained of feeling tired. Thyroid function testing was carried out on a Siemens Medical Solutions Ltd®, (Camberly, UK) immunoassay system. Results included a raised TSH of 45pg/ml (reference range 0.3 – 4.0) and a raised free T4 of 28.1 pmol/L (reference range 11.0 – 26). This combination of results was attributed to the patient taking her levothyroxine erratically. The patient denied any non-compliance. Repeat tests included a free liothyronine (T3) which was grossly elevated at 53.3 pmol/L (reference range 2.6 – 6.2). Further serological analysis identified thyroid hormone auto-antibodies which were causing falsely elevated free T4 and free T3 results. The patients levothyroxine dose was subsequently increased to 150mcg od and the patient felt improved with a reduced TSH of 8.2pg/ml. The full consequences of her under-treated hypothyroidism during her current pregnancy are as yet unknown. Simplified diagramatic representation of the Siemens immunoassay used to measure free T4. Step 1. - monoclonal murine anti-T4 antibody (Ab) present on assay beads is mixed with patient serum. Step 2. - free T4 (fT4) within patient serum binds to murine Ab Step 3. - a T4 analogue labelled with a chemiluminescent ligand is added and binds to any remaining available murine Ab. The amount of free T4 present with a patients serum is therefore inversely proportional to the amount of labelled analogue bound by the murine Ab. Antibodies present with a patient’s sera that interfere with binding of either free T4 or the labelled T4 analogue to the murine Ab can cause inaccurate results Discussion Patient mismanagement can occur when immunoassays are affected by antibody interference causing falsely elevated or reduced results. With the increasing use of immunoassays in the laboratory it is likely that similar case reports will become more common. Clinician awareness of artefactual interference in immunoassays needs to be raised and any results that appear to be incongruous with the clinical presentation should be discussed with the biochemistry team and evaluated for antibody interference.