A thyroid hormone action indicator transgenic mouse comprising stably integrated in its haploid genome a single copy of a recombinant DNA construct, useful for the assessment of thyroid hormone action, wherein the recombinant DNA construct comprises, in 5' to 3' direction, at least the following elements: - a thyroid hormone responsive segment, wherein the thyroid hormone responsive segment comprises a single copy or multiple copies of a thyroid hormone responsive element, wherein the TRE is a TRE of the dio 1 gene, and wherein said TRE is present in 1 to 5 copies, - a promoter operably linked to an expression enhancer, - a coding sequence encoding a reporter, wherein the reporter is a protein which, when expressed in a cell, said cell provides an optically detectable signal or which can be detected by an enzymatic assay, wherein - the promoter drives expression of the reporter in cells of one or more tissue(s) of said mouse, - a thyroid hormone receptor, when expressed in said cells, is expressed independently from the construct or is not expressed in said cells, - a TR, if present and a TH, if present, forms a complex in said cells, - said complex, if present, up-regulates expression of the reporter via the thyroid hormone responsive segment, whereby - the level of the expression of the reporter correlates with the level of said complex and - wherein the recombinant DNA construct is flanked by insulator sequences to protect the DNA construct from influences from neighbouring sequences, - wherein the promoter is a minimal promoter with minimal responsiveness to transcription factors beyond those that are required to form the basic transcriptional complex or the promoter is a minimal promoter lacking responsiveness to transcription factors beyond those that are required to form the basic transcriptional complex and the promoter is a ubiquitous promoter, and wherein the recombinant DNA construct is integrated into the genome of the mouse by a transposon based delivery. - a thyroid hormone responsive segment, wherein the thyroid hormone responsive segment comprises a single copy or multiple copies of a thyroid hormone responsive element, wherein the TRE is a TRE of the dio 1 gene, and wherein said TRE is present in 1 to 5 copies, - a promoter operably linked to an expression enhancer, - a coding sequence encoding a reporter, wherein the reporter is a protein which, when expressed in a cell, said cell provides an optically detectable signal or which can be detected by an enzymatic assay, - the promoter drives expression of the reporter in cells of one or more tissue(s) of said mouse, - a thyroid hormone receptor, when expressed in said cells, is expressed independently from the construct or is not expressed in said cells, - a TR, if present and a TH, if present, forms a complex in said cells, - said complex, if present, up-regulates expression of the reporter via the thyroid hormone responsive segment, whereby - the level of the expression of the reporter correlates with the level of said complex and - wherein the recombinant DNA construct is flanked by insulator sequences to protect the DNA construct from influences from neighbouring sequences, - wherein the promoter is a minimal promoter with minimal responsiveness to transcription factors beyond those that are required to form the basic transcriptional complex or the promoter is a minimal promoter lacking responsiveness to transcription factors beyond those that are required to form the basic transcriptional complex and the promoter is a ubiquitous promoter, and wherein the recombinant DNA construct is integrated into the genome of the mouse by a transposon based delivery.