A NOVEL MUTATION CAUSING COMPLETE THYROID BINDING GLOBULIN DEFICIENCY (TBG-CD MIA) IN A MALE WITH COEXISTING GRAVES DISEASE.

OBJECTIVE

An asymptomatic male was found on screening to have a low serum TSH and total T. The diagnosis of Graves' disease was made with positive thyroid stimulating immunoglobulin (TSI) and elevated free T in the presence of complete TBG deficiency (TBG-CD). Genetic testing of the patient and family members revealed a novel frameshift mutation in the () gene resulting in a complete deficiency of the protein.

METHODS

The laboratory testing included total T, free T by analog method and direct dialysis and TBG measurements. Sequencing of genomic DNA was performed from peripheral blood.

RESULTS

A 35-year-old East Indian male was referred to endocrinology because of abnormal thyroid function tests (TFTs): TSH 0.01 mIU/L (0.4-3.6), total T 3.0 µg/dl (5.5-10.5) done as part of a "routine office visit". Upon further testing, the serum free T 2.0 ng/dl (0.8-1.8) and TSI 355% (<140% baseline) were elevated and the diagnosis of Graves' disease was made. TBG deficiency was suspected because the total T concentration was inconsistent with hyperthyroidism and further testing confirmed TBG was undetectable. Sequencing of the gene revealed a novel hemizygous frameshift mutation: p.Ala64ProTer106, TBG-CD Mia (numbering excludes 20 a.a. signal peptide) associated with the complete deficiency of TBG in a patient with Graves' disease.

CONCLUSION

Patients with Graves' disease harboring a mutation have conflicting TFTs. If a clinically hyperthyroid patient presents with normal or low total T, serum TBG should be measured to identify an abnormality and prevent unnecessary testing.