We describe the cloning of a cDNA encoding TRH in Solea senegalensis. Phylogenetic analysis clustered it with other fish counterparts. Eight conserved TRH progenitors and highly divergent spacers were identified. Expression profiles of TRH were analyzed in juvenile tissues, during larval development, and in post-larvae grew under four photoperiods (12L:12D, 12D:12L, 24L:0D and 24D:0L) during four weeks using a real-time PCR approach. In juvenile fish, TRH mRNA was expressed ubiquitously although with the highest transcript levels in brain. During larval development, TRH mRNA levels were higher at 2-3days after hatching decreasing progressively until metamorphosis. Hormonal treatments using thiourea and T4 showed no regulation at transcriptional level by thyroid hormones. Moreover, no significant changes in mRNA levels between photoperiods at any time were detected. Overall, these results indicate that TRH do not participate in the regulation of HPT nor adaptation to photoperiod in sole.