A hyperthyroid patient with measurable thyroid-stimulating hormone concentration - a trap for the unwary.
BACKGROUND
In a patient with hyperthyroidism, the detection of elevated thyroid hormone concentration with measurable thyroid-stimulating hormone (TSH) value poses considerable diagnostic difficulties.


METHODS
This 38-year-old lady presented with clinical features of thyrotoxicosis. Her serum free thyroxine concentrations were unequivocally elevated [45 to 82 pmol/L (reference interval, 10 to 20 pmol/L)] but the serum TSH values were persistently within the reference interval [0.49 to 2.48 mIU/L (reference interval, 0.45 to 4.5 mIU/L)].


METHODS
Investigations excluded a TSH-secreting pituitary adenoma and a thyroid hormone resistance state and confirmed false elevation in serum TSH concentration due to assay interference from heterophile antibodies. The patient was treated with carbimazole for 18 months.


RESULTS
The heterophile antibody-mediated assay interference disappeared 10 months following the initiation of treatment with carbimazole, but returned when the patient relapsed. It disappeared again 2 months after the initiation of treatment.


CONCLUSIONS
Clinicians should be aware of the potential for interference in immunoassays, and suspect it whenever the test results seem inappropriate to the patient's clinical state. Misinterpretation of test values, arising as a result of assay interference, may lead to misdiagnosis, unnecessary and at times expensive investigations, delay in initiation of treatment and worst of all, the initiation of inappropriate treatment.
Tan
Mary  Jean
M
Department of Medicine, Tan Tock Seng Hospital, Singapore.
Tan
Florence
F
Hawkins
Robert
R
Cheah
Wei-Keat
WK
Mukherjee
J J
JJ
D000236:Adenoma / Q000175:diagnosis; D000328:Adult; D000910:Antibodies, Heterophile / Q000032:analysis / Q000276:immunology; D003951:Diagnostic Errors; D005260:Female; D006111:Graves Disease / Q000175:diagnosis*; D006801:Humans; D007118:Immunoassay; D010911:Pituitary Neoplasms / Q000175:diagnosis; D013971:Thyrotoxicosis / Q000097:blood / Q000175:diagnosis* / Q000276:immunology; D013972:Thyrotropin / Q000097:blood*; D013974:Thyroxine / Q000097:blood
D000910:Antibodies, Heterophile; D013972:Thyrotropin; D013974:Thyroxine