<P> Thyroid hormones (T and T) are produced by the follicular cells of the thyroid gland and are regulated by TSH made by the thyrotropes of the anterior pituitary gland . The effects of T in vivo are mediated via T (T is converted to T in target tissues). T is 3 - to 5 - fold more active than T . </P> <P> Thyroxine (3, 5, 3', 5' - tetraiodothyronine) is produced by follicular cells of the thyroid gland . It is produced as the precursor thyroglobulin (this is not the same as thyroxine - binding globulin (TBG)), which is cleaved by enzymes to produce active T . </P> <P> The steps in this process are as follows: </P> <Ol> <Li> The Na / I symporter transports two sodium ions across the basement membrane of the follicular cells along with an iodide ion . This is a secondary active transporter that utilises the concentration gradient of Na to move I against its concentration gradient . </Li> <Li> I is moved across the apical membrane into the colloid of the follicle . </Li> <Li> Thyroperoxidase oxidizes two I to form I. Iodide is non-reactive, and only the more reactive iodine is required for the next step . </Li> <Li> The thyroperoxidase iodinates the tyrosyl residues of the thyroglobulin within the colloid . The thyroglobulin was synthesised in the ER of the follicular cell and secreted into the colloid . </Li> <Li> Iodinated Thyroglobulin binds megalin for endocytosis back into cell . </Li> <Li> Thyroid - stimulating hormone (TSH) released from the adenohypophysis binds the TSH receptor (a G protein - coupled receptor) on the basolateral membrane of the cell and stimulates the endocytosis of the colloid . </Li> <Li> The endocytosed vesicles fuse with the lysosomes of the follicular cell . The lysosomal enzymes cleave the T from the iodinated thyroglobulin . </Li> <Li> The thyroid hormones cross the follicular cell membrane towards the blood vessels by an unknown mechanism . Text books have stated that diffusion is the main means of transport, but recent studies indicate that monocarboxylate transporter (MCT) 8 and 10 play major roles in the efflux of the thyroid hormones from the thyroid cells . </Li> </Ol>

Which of the following is not an action of t3 and t4