Pituitary gland thyroid hormones

As in insects, a complex interaction of hormones in the amphibian larva precipitates metamorphosis. Ultimately, two major classes of hormones act together to control amphibian metamorphosis the thyroid hormones (made by the thyroid gland) and prolactin (made by the pituitary gland). Thyroid hormones function somewhat like the molting hormones of insects, in that an increase of their concentration relative to prolactin leads to metamorphosis of the larva into the adult. Prolactin functions somewhat like the juvenile hormones of insects, in that it tempers the action of the thyroid hormones. In most species, thyroid hormones increase dramatically in concentration during metamorphosis and this stimulates resorption of certain larval organs and differentiation of new adult organs. 

The inner organs and their interactions are directed by the hormones of hormonic glands—pituitary gland, thyroid gland, suprarenal bodies, and... 

Thyroid autonomy appears as a solitary toxic nodule or toxic multinodular goitre. In toxic thyroid, the nodule s synthesis and secretion of thyroid hormones is autonomous from the thyroid-stimulating hormone (TSH), which is produced in the pituitary gland. Accordingly TSH is suppressed and the extranodular thyroid tissue is functionally downregulated. Thyroid autonomy occurs frequently in iodine-deficient countries, whereas it is much less common in iodine-sufficient areas. Constitu-tively activating mutations in the TSH receptor and in the Gs a protein are the major molecular aetiology of toxic thyroid nodules. 

A trophic hormone acts on another endocrine gland to stimulate secretion of its hormone. For example, thyrotropin, or thyroid-stimulating hormone (TSH), stimulates the secretion of thyroid hormones. Adrenocorticotropin, or adrenocorticotropic hormone (ACTH), stimulates the adrenal cortex to secrete the hormone cortisol. Both trophic hormones are produced by the pituitary gland in fact, many trophic hormones are secreted by the pituitary. The pituitary gland is sometimes referred to as the "master gland" because its hormones regulate the activity of other endocrine glands. 

Reports of the effects of Li+ upon the thyroid gland and its associated hormones are the most abundant of those concerned with the endocrine system. Li+ inhibits thyroid hormone release, leading to reduced levels of circulating hormone, in both psychiatric patients and healthy controls [178]. In consequence of this, a negative feedback mechanism increases the production of pituitary TSH. Li+ also causes an increase in hypothalamic thyroid-releasing hormone (TRH), probably by inhibiting its re-... 

TSH-induced hyperthyroidism is diagnosed by evidence of peripheral hypermetabolism, diffuse thyroid gland enlargement, elevated free thyroid hormone levels, and elevated serum immunoreactive TSH concentrations. Because the pituitary gland is extremely sensitive to even minimal elevations of free T4, a normal or elevated TSH level in any thyrotoxic patient indicates inappropriate production of TSH. 

In another factor related to the thyroid gland, wide variation has been observed with respect to the thyroid-stimulating hormone (TSH) of the pituitary. D Angelo8 and co-workers, using tadpoles as a tool, have studied the thyroid and TSH activity of the serums of normal and diseased individuals. Both limb growth (metamorphosis) induced by thyroid hormone and the development of thyroid tissue induced by TSH were measured. 

TSH, or thyrotropin, is a glycosylated protein of two subunits, a and p. TSH stimulates the thyroid gland to produce thyroid hormones. Deficiencies are treated by giving thyroxine itself rather than TSH, but TSH is available for diagnostic purposes to differentiate between pituitary and thyroid gland failure as causes of hypothyroidism (see Chapter 65). 

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