Thyroid function tests in several families with SBP2 deficiency studied in the authors’ laboratory. Grey regions indicate the normal range for the respective test. Affected individuals are represented as red squares and unaffected members of the families, as blue circles. B. In vivo studies: Serum TSH and corresponding serum T₄ and T₃ levels, before and during the oral administration of incremental doses of L-T₄ and L-T₃. Note the higher concentrations of T₄ required to reduce serum TSH in the affected subjects; C. In vitro studies: Deiodinase 2 enzymatic activity and mRNA expression in cultured fibroblasts. Baseline and stimulated D2 activity is significantly lower in affected. There is a significant increase of DIO2 mRNA with dibutyryl cyclic adenosine monophosphate [(db)-cAMP)], in both unaffected and affected (*p <0.001) while there are no significant differences in baseline (db)-cAMP stimulated DIO2 mRNA in affected versus the unaffected.