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Endocrine Abstracts (2022) 82 WC5 | DOI: 10.1530/endoabs.82.WC5

Norfolk and Norwich University Hospital, Norwich, United Kingdom

Case 1: A 17-year-old female was referred to the endocrinology outpatients due to abnormal thyroid function tests (TFTs) (as below) detected on routine monitoring for Thyroxine replacement therapy. Following exclusion of pregnancy, possibilities of assay interference due to heterophilic antibodies as well as thyroid hormone resistance were considered. Repeat analysis was arranged at two laboratories using different methods. Results were concordant, excluding assay interference. The above results were explained by intermittent non-compliance with replacement therapy which is a common cause for erratic TFTs due to different half-lives of thyroid hormones as well as exogenous Thyroxine. Intermittent Thyroxine intake can result in normal or elevated thyroid hormone levels which fail to normalise the TSH levels.

TestResultReference interval
TSH23.55 mU/l0.35-3.50
Free T413.5 pmol/l7.5-21.5
Free T34.7 pmol/l3.8-6.0
Anti-TPO Ab>600 kU/l0.0-34.0
TestResultReference interval
TSH (Abbott)17.12 mU/l0.35-3.50
Free T4 (Abbott)13.1 pmol/l7.5-21.1
TestResultReference interval
TSH13.71 mU/l0.35-3.50
Free T433 pmol/l7.5-21.5
Free T36.33.8-6.0
TestResultReference interval
TSH46.20 mU/l0.35-3.50
Free T45 pmol/l7.5-21.5
TestResultReference interval
TSH23.9 mU/l0.35-3.50
Free T45.0 pmol/l7.5-21.5
Free T31.6 pmol/l3.8-6.0

Case 2: An 87-year-old woman with recently diagnosed “hypothyroidism” was referred to the endocrine outpatients due to discrepant TFTs. Historic results confirmed a persistently raised TSH for two years with free T4 levels at the lower end of normal range. Recent initiation of Thyroxine replacement therapy had led to thyrotoxicosis symptoms without any change in TSH levels. A spuriously elevated TSH was suspected. Repeat TFTs (as below) check with a 9 a.m. anterior pituitary hormone profile (normal) was undertaken. A trial off Thyroxine for 3 months resulted in the following TFTs: Given above TFTs, Thyroxine replacement therapy was resumed with repeat testing as follows: The patient was investigated for macro TSH using polyethylene glycol (PEG) precipitation method revealing a “biologically active” TSH of approximately 3 mU/l, which was within the reference interval. MacroTSH is an uncommon cause for interference which can lead to spuriously elevated results. Going forward for this patient, free T4 should be used to assess for adequacy of replacement therapy.

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