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CASE REPORT
Elevated free thyroxine and non-suppressed thyrotropin
  1. Shao Feng Mok1,
  2. Tze Ping Loh2,
  3. Byrappa Venkatesh3,
  4. Doddabele Srinivasa Deepak1
  1. 1Department of Medicine, National University Hospital, Singapore, Singapore
  2. 2Department of Laboratory Medicine, National University Hospital, Singapore, Singapore
  3. 3Department of Comparative and Medical Genomics Lab, Institute of Molecular and Cell Biology, A*STAR, Singapore, Singapore
  1. Correspondence to Dr Tze Ping Loh, lohtp{at}yahoo.com

Summary

A young man was diagnosed with hyperthyroidism 10 years prior to current presentation after a random health screening revealed an elevated free thyroxine (fT4) of 36.9 pmol/L. During that time, he saw multiple physicians and was treated with carbimazole intermittently. His repeat thyroid function tests showed persistently elevated fT4 ranging 25–35.7 pmol/L and non-suppressed thyroid-stimulating hormone (TSH) concentrations of 6.78–22.1 mIU/L. He had a smooth, firm and non-tender goitre. At our institution, laboratory interference was first excluded by serial dilution study (TSH) and retesting of TSH and fT4 on alternate assay, which gave reproducible results. His normal α-subunit and sex hormone binding globulin, partially suppressed TSH by high dose triiodothyronine (T3), and positive TSH response to thyrotropin-releasing hormone stimulation were consistent with resistance to thyroid hormone syndrome. The diagnosis was confirmed by direct sequencing of thyroid hormone receptor-β gene, revealing a heterozygous R320 L mutation that causes reduced T3 affinity and reduced corepressor dissociation.

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