Inherited thyroxine binding globulin (TBG) disorder can be identified incidentally or through neonatal screening test. TBG excess is characterized by high levels of thyroxine (T4) but normal level of free T4 (fT4), while TBG deficiency presents with low T4 levels and normal fT4 levels. TBG excess is caused by TBG gene duplication or triplication. A 27 day old newborn was brought to the hospital because of hyperthyroxinemia that was identified through neonatal screening test. His mother had no history of any thyroid disease. His fT4 and thyroid stimulating hormone (TSH) levels were 1.99 ng/dL (0.902.30) and 4.54 mIU/L (0.506.50), respectively. His serum total triiodothyronine (T3) level was 322.50 ng/dL (105.00345.00). His TBG level was 68.27 mg/L (16.0036.00), and his thyroid autoantibodies were all negative. At 6 months of age, his serum T3 and T4 levels were 322.10 ng/dL (105.00245.00) and 16.70 ng/dL (5.9016.00), respectively. His TSH and fT4 levels were 2.68 mIU/L (0.506.50) and 1.71 ng/dL (0.802.10), respectively, and his TBG level was 48.77 mg/L (16.0036.00). At 12 months of age, his serum T3 and T4 levels were 333.30 ng/dL (105.00269.00) and 21.30 ng/dL (7.3015.00), respectively. His TSH, free T3 (fT3) and fT4 levels were 2.69 mIU/L (0.608.00), 6.48 ng/dL (2.805.20) and 1.82 ng/dL (0.802.00), respectively. His T3 uptake value was 19.1% (2737), and TBG level was 50.20 mg/L (14.0028.00). Hormonal studies showed consistently elevated T3 and T4 levels but normal fT4 and TSH levels. Concentrations of TBG were two to three times higher than normal values. His growth and development were normal. TBG excess is considered a clinically euthyroid condition that requires no treatment. TBG excess should be considered as a potential differential diagnosis for hyperthyroxinemia and especially high T3 levels.
10 - 12 Sep 2016
European Society for Paediatric Endocrinology