Loh TP, Kao SL, Halsall DJ, Toh SAES, Chan E, Ho SC, Tai ES, Khoo CM. Macro-thyrotropin: a case report and review of literature.
J Clin Endocrinol Metab 2012;
97:1823-8. [PMID:
22466337 DOI:
10.1210/jc.2011-3490]
[Citation(s) in RCA: 52] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
CONTEXT
Isolated elevation of TSH in the absence of thyroid symptoms can be very rarely caused by a macromolecule formed between TSH and Ig (macro-TSH), confounding the interpretation of thyroid function test results.
OBJECTIVE
We described the use of several commonly available laboratory-based approaches to investigate an isolated TSH elevation [232 mIU/liter; free T(4), 10 pmol/liter (reference interval, 10.0-23.0 pmol/liter), Vitros platform] in a clinically euthyroid elderly gentleman, which led to the diagnosis of macro-TSH.
METHODS AND RESULTS
TSH concentration of the patient was significantly lower (122 mIU/liter) when measured on the Advia Centaur platform. Serial dilution of the patient's sample showed a nonlinear increase in TSH recovery at increasing dilution (nonlinearity). Polyethylene glycol precipitation and mixing the patient's sample with a hypothyroid patient sample showed reduced TSH recovery, suggesting the presence of a high molecular weight interfering substance and excess TSH binding capacity, respectively. Heterophile blocking tube studies and rheumatoid factors were negative. Gel filtration chromatography demonstrated a TSH peak fraction that approximated the molecular size of IgG; together with the excess TSH binding capacity, this indicated the presence of TSH-IgG macro-TSH. A review of 12 macro-TSH case reports showed that samples with macro-TSH produce over-recovery with dilution, return negative results on anti-animal and anti-heterophile blocking studies, and commonly have recovery of less than 20% when subjected to polyethylene glycol precipitation.
CONCLUSION
Macro-TSH is an underrecognized laboratory interference. Routine laboratory techniques described above can help diagnose this rare entity. A close dialogue between the physician and the laboratory is important in approaching such cases.
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