In knockout embryos development and/or differentiation of the thyroid gland appears unaffected. In contrast, soon after birth, Dicer knockout mice develop severe hypothyroidism, accompanied by a progressive derangement of thyroid follicular structure, thus showing that Dicer function is mandatory for normal thyroid function in later life. To the best of our knowledge, the data presented here demonstrate for the first time that a normal miRNA metabolism is essential for maintaining the normal thyroid gland structure and function in the adult. In this paper we demonstrate that the micro-RNA processing enzyme Dicer is essential for several aspects of postnatal thyroid gland function and structure. Mice with thyroid-specific ablation of Dicer develop a pronounced impairment of thyroid function leading to shortened life span. The observed high serum TSH level and reduced body weight are indicative of severe hypothyroidism in the knockout mice. Interestingly, no gross alterations of thyroid gland localization, morphology and differentiation are observed during embryonic development and in newborn mice. Dicer conditional knockout mice have been obtained using Pax8Cre/+ mouse line, in which Cre recombinase is controlled by the Pax8 locus, that is active in the in thyroid from E8.5 and throughout the adult life. Thus, Dicer-dependent miRNAs are supposed to be ablated from E8.5 and consequently also during migration of the thyroid bud from the R428 pharyngeal floor to its final position in front of the trachea, as well as during the completion of thyrocyte differentiation. Although we cannot exclude that the rate of Cre-mediated Dicer inactivation at early developmental stages might not be sufficient to completely abolish the expression of miRNAs, these data argue for the possibility that miRNAs are dispensable during initial stages of thyroid morphogenesis. Molecular analysis of knockout thyroid glands at 1 month after birth shows a strongly reduced expression of late differentiation markers such as Nis and Tg, while the expression of the early differentiation markers Pax8 and Nkx2.1 appears unaffected. Furthermore the epithelial polarity markers Cdh1 and Cdh16 are downregulated, thus correlating with the loss of polarized follicular architecture. As it has been previously demonstrated that thyroidspecific Cdh1 knockout mice still retain thyrocyte polarization, the reduction of Cdh1 membrane localization might be a consequence, rather than causative, of the compromised cell polarity. Hypothetically also loss of Nis protein expression could be a consequence of altered thyrocyte polarity. It is well known that Pax8 and Nkx2.1 transcription factors positively regulate Tg transcription in vitro, and that both are necessary for Tg expression in vivo. The existence of a developmental stage between E8.5 and E15.5 in which Tg is still absent despite the presence of both transcription factors, however, demonstrates that Pax8 and Nkx2.1 are not sufficient to switch-on the expression of Tg.