Figure 4. Thyroid hormone signaling *Follicular cells contain thyroid stimulating hormone receptors (TSHR). Once bound to the hormone TSH, this G-protein coupled receptor generates the second messenger cyclic AMP. This second messenger activates protein kinases that trigger signal cascades responsible for thyroid cell proliferation and function. TSH stimulation of TSHR also leads to iodide transport into the cell via the sodium iodide symporter. This iodide is oxidized and then bound to tyrosine residues on thyroglobulin (TG), a process catalyzed by thyroperoxidase. This primed form of TG can then be cleaved through proteolysis in the cell to generate the thyroid hormones T3 and T4.