Looking for the Mechanism of Action of Thyroid Hormone
Figure 3
Three hypothetical models for thyroid hormone action depicting permutations of multiple hormonal interplay at the postreceptor level, represented by “Black Boxes” A, B, and C. According to model 1, the simplest situation is that T3 interacting with a single thyroid hormone receptor (TR) modifies the activity of a key postreceptor complex which gives rise to a chain of multiple responses . The multiplicity of responses to a hormone may also arise from different chemical or structural isoforms of a single receptor, represented in model 2 as TR, R2 and R3 together giving rise to multiple actions through functionally different Black Boxes. Alternatively, the same single receptor is present in different cellular locations and behaves effectively as different receptors (TR, R2, R3) in the same or different cells. In model 3, the multiplicity is a function of another hormone or signal (H2, H3), acting via different receptors (R2, R3) to modify the nature, multiplicity, or extent of the action(s) of T3, either before or after the action at the level of Black Boxes. The multiplicity, can also result from further downstream interactions of the postreceptor responses (e.g., ).
