Hypothalamus thyroid-releasing hormone

Thyroid disorders may be divided into over- and underproduction of the thyroid hormones. These may be caused by thyroid gland disorders or disorders of the pituitary gland (TSH production) or hypothalamus (thyrotropin-releasing hormone release). Thyroid hormone deficiency in infancy may cause mental retardation if it is not corrected immediately after birth. For this reason, many states require thyroid function tests in all newborns. In adults, thyroid deficiency may be caused by Hashimoto s thyroiditis, an immune disorder, or dietary iodine deficiency, in which case it is called simple goiter. The term "myxedema" has been used to refer to hypothyroidism of whatever cause. Myxedemas may... 

Hormones secreted in the hypothalamus, which releases pituitary hormones, are believed to bind to specific receptors of the membranes of the pituicyte. This action is dependent upon the presence of Ca Therefore, it has been proposed that the hormone depolarizes the cell membrane increasing the permeability to calcium which would, in turn, release the hormone. Whatever the role of calcium, it is known in the case of thyroid releasing hormone that it stimulates adenylcyclase and cAMP. 

Hypothalamus Oxytocin, vasopressin, thyroid releasing hormone, LH-RH, somatostatin... 

Somatostatin inhibits secretion of growth hormone and other hormones such as prolactin from the anterior pituitary and is widely distributed in the brain in interneurons and projection pathways. All parts of the cortex contain local circuit somatostatin positive neurons, concentrated in layers V and VI, as does the amygdala and striatum. The nucleus accumbens and adjacent ventral putamen and caudate—designated limbic striatum—have particularly high concentrations of fibres. By contrast, TRH which regulates release of thyroid stimulating hormone and prolactin by the pituitary is generally confined to nuclei in and around the hypothalamus. 

The secretion of anterior pituitary hormones is controlled in part by hypothalamic regulatory factors that are stored in the hypothalamus and are released into the adenohypophyseal portal vasculature. Hypothalamic regulatory factors so far identified are peptides with the exception of dopamine. Secretion of anterior pituitary hormones is also controlled by factors produced more distally that circulate in the blood. Predominant control of hormone production may be relatively simple, as with thyroid-stimulating hormone (TSH), the production of which is primarily stimulated by thyrotropin-releasing hormone (TRH) and inhibited by thyroid hormones, or it may be complex, as with prolactin, the production of which is affected by many neurotransmitters and hormones. 

Control of thyroid function via thyroid-pituitary feedback is also discussed in Chapter 37. Briefly, hypothalamic cells secrete thyrotropin-releasing hormone (TRH) (Figure 38-3). TRH is secreted into capillaries of the pituitary portal venous system, and in the pituitary gland, TRH stimulates the synthesis and release of thyrotropin (thyroid-stimulating hormoneTSH). TSH in turn stimulates an adenylyl cyclase-mediated mechanism in the thyroid cell to increase the synthesis and release of T4 and T3. These thyroid hormones act in a negative feedback fashion in the pituitary to block the action of TRH and in the hypothalamus to inhibit the synthesis and secretion of TRH. Other hormones or drugs may also affect the release of TRH or TSH. 

Endogenous thyroid hormone production begins when neuro-input tells the hypothalamus to synthesize an release Thyroptropin -Releasing -Hormone. (TRH)... 

Thyroid hormone production is controlled by the hypothalamic-pituitary system (see Chapter 28). Thyrotropin-releasing hormone (TRH) from the hypothalamus stimulates the release of thyroid-stimulating hormone (TSH) from the anterior pituitary.17,63 TSH then travels via the systemic circulation to the thyroid gland to stimulate the production of thyroxine and triiodothyronine. 

Thyrotropin-releasing hormone, or protirelin, is a tripeptide hormone found in the paraventricular nuclei of the hypothalamus as well as in other parts of the brain. TRH is secreted into the portal venous system and stimulates the pituitary to produce thyroid-stimulating hormone (TSH, thyrotropin), which in turn stimulates the thyroid to produce thyroxine (T4) and triiodothyronine (T3). TRH stimulation of thyrotropin is blocked by thyroxine and potentiated by lack of thyroxine. 

Thyrotropin, thyroliberin, or thyroid-stimulating hormone (TSH) is a peptide released by the anterior pituitary gland that stimulates the thyroid gland to release thyroxine (Ladram et al., 1994). The release of TSH is triggered by the action of thyrotropin-releasing faetor (TRF), a peptidic substance found in the hypothalamus of the brain and influencing the secretion of glandula thyroidea. 

The pituitary gland is involved in feedback regulation of thyroid activity High levels of T4 in the bloodstream result in inhibition of TSH secretion. Low levels of T4 result in an increase in TSH secretion. These effects are dependent on the conversion of T4 to T3 within the pituitary, fhe activity of the pituitary is controlled by thyrotropin-releasing hormone (TRH), a hormone synthesized in the hypothalamus. TRH is a tripeptide with the structure pyroglutamate-histidine-proline-NH . Note the C-terminal amide group, which is required for the activity of many peptide hormones. TRH stimulates the synthesis and secretion of TSH. Apparently TRH is involved in regulating the sensitivity of the pituitary to the inhibitory feedback control mechanism mentioned earlier. 

Secondary hypothyroidism Either the hypothalamus is unable to release sufficient quantities of thyrotropin-releasing hormone or the anterior pituitary gland is unable to release sufficient thyroid-stimulating hormone. 

Secondary hyperthyroidism Either the hypothalamus releases a high quantity of thyrotropinreleasing hormone or the anterior pituitary gland releases a high quantity of thyroid-stimulating hormone. 

Three possible causes of hypothyroidism are the decreased secretion of thyrotropin-releasing hormone (TRH) by the hypothalamus gland, decreased secretion of thyroid-stimulating hormone (TSH) by the pituitary gland, and decreased secretion of thyroid hormones by the thyroid gland. 

Thyrotropin-releasing hormone (TRH)—A trophic hormone released by the hypothalamus that stimulates release of thyroid-stimulating hormone (TSH). 

TRH Thyrotropin-releasing hormone a tripeptide hormone that is released by the hypothalamus and that acts on the anterior pituitary to stimulate the release of thyroid-stimulating hormone. 

Thyroid hormone production is governed by the actions of the hypothalamic-pituitary-thyroid axis (HPT axis). Thyrotropin releasing hormone (TRH) synthesized by the hypothalamus is transported to the anterior pituitary, where TRF stimulates thyrotrophic cells to produce thyrotrophin (TSH). TSH is a glycoprotein with a short half-life of less than 20 min it is one of three pituitary glycoprotein hormones with alpha- and beta-subunits. These three pituitary glycoproteins (luteotrophin, fol-liotrophin, and TSH) share the same alpha-subunit. 

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