MicroRNAs (miRNA) are small non-coding RNAs that are post-transcriptionally regulating the gene expression. Dicer1 is a key enzyme influencing the miRNA processing. Tissue-specific silencing of Dicer1 provides a tool to study the influence of RNA interference in the tissue development and function. So far, little is known about miRNA in the development and function of the thyroid gland, but several miRNA genes are known to play a role in thyroid cancer. To understand the role of Dicer1 in thyroid in vivo, we have generated a thyroid-specific Dicer1 knockout mouse line by crossing Dicer1 fl/fl mice with Tg-Cre mice expressing the Cre-recombinase under the thyroglobulin promoter. These mice were shown to lack Dicer1 in thyrocytes, and as a consequence, showed an altered histology of the thyroid gland with a decreased formation of thyroid follicles. Furthermore, the mice developed hypothyroidism with a 2-fold increased thyroid stimulating hormone (TSH) bioactivity and 50 % reduced free thyroxine (fT4) serum levels compared to the wild-type mice. However, Dicer1 KO mice did not respond to goiter-inducing drug treatment (no significant change of thyroid weight) while control mice showed a 4-fold increased weight of the thyroid gland. The data, thus, indicate that Dicer1 plays a crucial role in thyroid function. In addition, we are in process of generating an inducible thyrocyte-specific Dicer1 KO model by applying a mouse line expressing a tamoxifen-inducible Cre under the thyroglobulin promoter. Characterizing the constitutive and inducible Tg-Cre Dicer1 KO mice will provide us with novel information about the role of Dicer1 in the development and function of the thyroid gland.