Thyroid hormones and the control of cell proliferation or cell differentiation: paradox or duality?

Amphibian metamorphosis perfectly illustrates a key paradox: thyroid hormones control diverse cellular processes depending on the tissue context. This point is also reinforced by a recent accumulation of evidence. For example, thyroid hormones and their nuclear receptor TRs have been described to function in different systems in synergy and/or in antagonism with other signaling pathways. This interaction helps explain their pleiotropic roles. This review summarizes the most important advances in this field, focusing in particular on the key action of thyroid hormones in controlling the balance between the processes of cell proliferation and cell differentiation in a few organs, with special attention paid to the intestine. We highlight similarities between the cellular and molecular events occurring during postnatal intestinal maturation at metamorphosis in amphibians, and comparable events observed at weaning in mice.