In situ hybridisation of TRH mRNA in the paraventricular nucleus of a hypothyroid rat.
The paraventricular nucleus on the left has been injected with a minute pellet of T3, whereas an inert substance was injected into the right nucleus. The striking suppression of the hybridisation signal indicates that T3 acted locally to suppress synthesis of mRNA and is thus indicative of a hypothalamic locus for the suppression of the hypothalamic-pituitary-thyroid axis. By courtesy of Endocrinology and Dr R Lechan.
