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Thyroid hormones (THs) are important regulatory factors of vertebrategrowth and development. Although the main TH secreted by the thyroid gland is theprohormone T4, the majority of TH actions depend on the binding ofthe active hormone T3 to its nuclear receptors. Appropriate T3availability in developing cells is regulated by iodothyronine deiodinases(D1-D3), the enzymes that activate and inactivate THs. Despite their essentialrole in TH regulation, deiodinases have received little attention in relationto their function during development. In this thesis we have used morpholinoknockdown in the zebrafish as a model to examine the impact of deiodinasedeficiency on early development, with the focus on the TH-inactivatingdeiodinase D3. Due to a wholegenome duplication in the earlyevolution of ray-finned fishes, zebrafish posses two paralogs for D3, dio3a and dio3b. We showed the expression of both genes during zebrafishembryonic and early larval development. Duringmost of this period, significantly higher levels of dio3b mRNA were detected compared to dio3a mRNA. In line with this, the contribution of dio3b to embryonic D3 activity seemed tobe predominant and the phenotype of dio3bmorphants was more severe and persisted longer compared to dio3a morphants. To compare thefunctions of the TH-activating and TH-inactivating pathways in zebrafishdevelopment, we examined the effects following co-knockdown of the activatingdeiodinases (D1 and D2) with those following knockdown of D3. We found that dio1dio2 morphants, as well as dio3b morphants, showed a delayedincrease in body length, eye size and ear size, delayed hatching, and disturbedswim bladder inflation. For all these effects the phenotype was more severe andpersisted longer in dio3b morphants.We further found some phenotypic abnormalities in dio3b morphants that were not present in dio1dio2 morphants. More specifically, dio3b knockdown seemed to cause a disorganized layer formation inthe swim bladder, and caused a reduced size in other endoderm derived organssuch as liver and intestine. dio3bmorphants also showed defects in embryonic and larval motility, and anupregulation of transcription in one of the genes involved in fast muscle development (mylpfa). To verify whetherthe developmental changes that follow dio3bknockdown are truly caused by excessive T3 availability due todecreased D3 activity, we used two approaches. First, we demonstrated that exposure toexogenous T3 caused similar developmental defects as seen in dio3b morphants. Next, we showed thatinjection of human DIO3 mRNA could substantiallyrescue D3 activity as well as the observed phenotype. Takentogether, we have demonstrated that maintaining a critical balance betweenTH-activation and TH-inactivation is important for development to proceednormally, and showed that zebrafish is an excellent model to further explorethe role of deiodinases during early vertebrate development.