Selenocysteine insertion sequence binding protein 2, SBP2 (SECISBP2), is required for selenoprotein synthesis. Partial SBP2 deficiency syndrome manifests characteristic thyroid function tests. The Sbp2 deficiency mouse model, Sbp2 iCKO, replicates this thyroid phenotype and was used for pathophysiologic investigations. As selenoproteins have antioxidative role in thyroid gland function, their deficiencies have potential to affect thyroid hormone (TH) synthesis. Sbp2 iCKO mice had larger thyroids relative to body weight and increased thyroidal T4 and T3 content while 5’ deiodinases enzymatic activities were decreased. Possible mechanisms for the discrepancy between the increased thyroidal T3 and normal circulating T3 were investigated in dynamic experiments. Treatment with bovine TSH resulted in increased delta T4 in Sbp2 iCKO mice, indicating increased availability of preformed thyroidal TH. Next, the recovery of TH levels was evaluated after withdrawal of chemical suppression. At one day, Sbp2 iCKO had higher serum and thyroidal T3 concomitant with lower TSH, confirming increased capacity of TH synthesis in Sbp2 deficiency. Decreased TH secretion was ruled out, as serum and thyroidal TH were high in Sbp2 iCKO mice. Treatment with low-iodine diet also ruled out thyroidal secretion defect as both serum levels and thyroidal TH content similarly declined over time in Sbp2 deficient mice compared to Wt. This study provides evidence for unsuspected changes in thyroid gland that contribute to the thyroid phenotype of Sbp2 deficiency. The increased thyroidal T4 and T3 content, when circulating T3 levels are normal is due to an increased TH synthesis capacity while thyroidal secretion is preserved.