In the present study, we report the uncommon case of a 9.6-yr-old girl with circulating an-ti-T3 autoantibodies (T3-Ab) and hyperthyroidism due to inappropriate secretion of TSH (IST). The diagnosis of IST was based on the findings of normal TSH levels (2.4 mU/L) in the presence of high free T4 (28.2 pmol/L) and free T3 (FT3) levels, as measured by direct measurement methods based on “one-step” analog tracer (28.0 pmo/L) and “two-step” Lisophase® (13.3 pmo/L) techniques. The discrepancy between the two measurements suggested a methodological interference due to T3-Ab in “one-step” technique, being the “two-step” methodology unaffected by the presence of such autoantibodies. T3-Ab were documented by high nonspecific binding of serum to labeled T3 (38.0% vs 4.3 ± 2.1% in controls). The clinical picture of hyperthyroidism, the qualitatively normal TSH responses to TRH and T3 suppression tests, the normal pituitary imaging and the values of some parameters of peripheral thyroid hormone action compatible with hyperthyroidism indicated that the patient was affected by pituitary resistance to thyroid hormones (PRTH). Chronic treatment with dopaminergic agent bromocriptine (7.5 mg/day) did not cause TSH secretion to be suppressed, while the administration of thyroid hormone analog TRIAC (1.4 mg/day) inhibited TSH release (from 2.4 to 0.2 mU/L). As a consequence, circulating thyroid hormone levels normalized and euthyroidism was restored. During TRIAC administration, FT3 levels, measured by “one-step” analog tracer technique, gave spuriously high values due to the methodological interference of T3-Ab (15.2 vs 4.3 pmol/L as measured by “two-step” Lisophase® technique). In conclusion, the present study indicates that a) the levels of circulating free thyroid hormones should be measured by methods unaffected by the presence of anti-iodothyronine autoantibodies in order to prevent either misdiagnosis of IST or, as in the present case, misinterpretation of the efficacy of TSH suppressive treatment, and b) it is mandatory to rule out the presence of such autoantibodies whenever IST is suspected.
- anti-iodothyronine autoantibodies, bromocriptine
- pituitary resistance to thyroid hormones
ASJC Scopus subject areas
- Endocrinology, Diabetes and Metabolism