77 year old gentleman was referred to endocrine clinic at Manchester Royal Infirmary in June 2017 due thyrotoxicosis with TSH 0.10 mu/l, free T4 24.3 pmol/l, and free T3 15.7 pmol/l. He was presented at that time with symptoms of tiredness, fatigue and tremor. He was initially treated with antithyroid drugs from June 2018 till May 2018 with radioactive iodine 579 MBq in May 2018. Subsequently, he developed symptoms of excessive tiredness, lack of energy, weight gain and cold intolerance which raised the possibility of post radioiodine hypothyroidism. His thyroid function test in August 2018 showed TSH 5.8 mu/l, free T4 8.0 pmol/l and free T3 9.9 pmol/l and he was commenced on levothyroxine 50 mcg daily. His repeated thyroid function in November 2018 revealed TSH 3.5 mu/l, and free T4>100.0 pmol/l. However, clinically he remained hypothyroid with symptoms of weight gain, fatigue and cold intolerance. Levothyroxine has been stopped temporarily and his repeated thyroid function test in January 2019 showed TSH 5.9 mu/l, free T4 38.8 and free T3 8.5. There was a clear discrepancy between clinical status and thyroid function test. Moreover, there was an evident of discordant (paradoxical free T4 and TSH levels) suggesting acquired assay interference. Therefore, a sample was sent to Salford Royal Hospital and the result was:
The results from Salford Royal Hospital seem correlate clinically and suggest that there has been Roche assay interference. He was recommenced on levothyroxine treatment in January 2019 and the dose has been optimized to 125 mcg daily with good clinical response and normalisation of TSH. Immunoassay interference is a well described phenomenon. Clinicians need to be aware of the potential for assay interference since it may lead to inappropriate treatment. Normal reference range: TSH (0.25.0 mu/l), Free T4 (924 pmol/l), Free T3 (3.66.4 pmol/l).
|Sample||Manchester Royal Hospital||Salford Royal Hospital|
|Free T4 (pmol/l)||38.8||<1.3|
|Free T3 (pmol/l)||8.5|||