PDE8B is a phosphodiesterase enzyme that hydrolyzes the second messenger cAMP, functioning as a negative regulator of cAMP/PKA signal transduction 1. The enzyme exhibits specific and abundant expression in thyroid tissue as a ~4.2 kb mRNA, with cAMP phosphodiesterase activity that is relatively resistant to common PDE inhibitors except dipyridamole 1. PDE8B plays a critical role in thyroid hormone regulation: genetic variants in PDE8B are robustly associated with TSH levels and show reciprocal changes with free T4 levels in longitudinal studies, though these associations are lost in individuals on levothyroxine replacement 2. Beyond thyroid function, PDE8B has broader physiological significance. In the heart, PDE8B is upregulated in chr5 atrial fibrillation, where it directly interacts with the Cav1.2α1C subunit to reduce L-type calcium current in an arrhythmogenic mechanism 3. In ovarian tissue, PDE8B inhibition prevents primordial follicle activation 4. Clinically, PDE8B alterations are associated with primary pigmented nodular adrenocortical disease through dysregulation of cAMP-dependent PKA signaling, where inactivating mutations predispose to adrenocortical tumors causing Cushing syndrome 567.