Posts Tagged ‘thyroid gland’

Reporter and Curator: Dr. Sudipta Saha, Ph.D.

Thyroid hormone (TH) signaling plays an important role in development and adult life. Many organisms may have evolved under selective pressure of exogenous TH, suggesting that thyroid hormone signaling is phylogenetically older than the systems that regulate their synthesis. Therefore, the negative feedback system by TH itself was probably the first mechanism of regulation of circulating TH levels. Neuroendocrine signalling allows for integration of function of distinct tissues in complex organisms, leading to coordinated response to a given challenge and increased fitness for that organism. The hypothalamic-pituitary-thyroid (HPT) axis is a classical example of how a neuroendocrine system regulates distinct functions of an organism both during development and in adult life in response to a variety of challenges, presumably improving its chance of success. For instance, thyroid function and circulating thyroid hormones (TH) levels change in response to some of the most demanding conditions an adult organism may be exposed to, such as reduced food availability, decreased environmental temperature, and illness. Interestingly, the presence of TH precedes the thyroid itself, and exogenous TH has major effects even on organisms that lack thyroid-like structures. Indeed, it has been hypothesized that some invertebrates may obtain TH from diet, suggesting that TH signalling is phylogenetically older than the systems that regulate their synthesis in multicellular organisms. Thus, it is tempting to hypothesize that the regulatory mechanisms that control TH synthesis evolved under the selective pressure of TH action. Indeed, it is well known that an excess of TH suppress, whereas the absence stimulates their own synthesis in a variety of organisms, including humans. Thus, it is plausible to assume that a negative feedback system was probably the first mechanism of regulation of TH levels. However, through evolution, new pathways emerged to control TH levels. In humans and other vertebrates, it is well known that TH negatively regulates its own production through central actions that modulate the hypothalamic-pituitary-thyroid (HPT) axis. Indeed, primary hypothyroidism leads to the up-regulation of the genes encoding many key players in the HPT axis, such as TRH, type 2 deiodinase (dio2), pyroglutamyl peptidase II (PPII), TRH receptor 1 (TRHR1), and the TSH a- and b-subunits. However, in many physiological circumstances, the activity of the HPT axis is not always a function of circulating TH concentrations. Indeed, circadian changes in the HPT axis activity are not a consequence of oscillation in circulating TH levels. Similarly, during reduced food availability, several components of the HPT axis are down-regulated even in the presence of lower circulating TH levels, suggesting the presence of a regulatory pathway hierarchically higher than the feedback system.

Regulation of the HPT axis is complex, and every year new advances in the area are made. However, it is far from fully understanding its control. Undoubtedly, the negative feedback imposed by TH plays a role in the regulation of the HPT axis, but there are clearly other key pathways that are working to keep TH levels adequate. Indeed, under physiological conditions, feedback regulation seems to play a less relevant role when compared with conditions where primary dysfunction of the thyroid gland is present. It is true that in some situations (e.g. starvation), changes in central action of TH might cause a shift in the set point of the HPT axis. However, the signaling pathways driving these putative set-point-modifying phenomena need to be elucidated. For instance, it is known that the coregulators SRC-1 and NCoR1 (nuclear receptor corepressor 1) control the action of TH also on negatively regulated genes and those changes in their expression/ action shift the set point of the HPT axis. However, it remains to be demonstrated how this is orchestrated in physiological conditions and what would be driving these modifications. Neural circuitries regulate thyroid activity through the control of TRH release in the median eminence, and this seems to be especially relevant in the control of circadian rhythm and in response to both fasting and reduced environmental temperature. Interestingly, during those situations, changes in circulating TH levels do not elicit a counter-regulatory response of the hypothalamic-pituitary axis. Therefore, it is tempting to assume the existence of regulatory mechanisms able to override negative feedback regulation. Strikingly, some of these pathways may be controlling distinct responses to a common stressor, such as during restricted food availability. In that situation, NPY (neuropeptide Y) signaling plays a crucial role in the control of both food intake and HPT axis activity, suggesting that these pathways may have evolved together as a common energy replenishing response. Taken together, this suggests that the regulation of the HPT axis occurs at multiple levels and is highly integrated with the internal milieu and the external environment.

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