Thyroid gland regulation

Biologic Effect of the Thyroid Hormone. The thyroid gland regulates the entire metabolism of the adult organism. Disturbances of the thyroid function are reflected in the basal metabolic rate (metabolism while resting, cf. Chapt. XXII-1). In hyperthyroidism (thyrotoxicosis) the rate is elevated during hypofunction (clinical manifestation myxedema), it is lowered. This phenomenon is used frequently in judging thyroid function. Administration of thyroxine raises the basal... 

Three hormones regulate turnover of calcium in the body (22). 1,25-Dihydroxycholecalciferol is a steroid derivative made by the combined action of the skin, Hver, and kidneys, or furnished by dietary factors with vitamin D activity. The apparent action of this compound is to promote the transcription of genes for proteins that faciUtate transport of calcium and phosphate ions through the plasma membrane. Parathormone (PTH) is a polypeptide hormone secreted by the parathyroid gland, in response to a fall in extracellular Ca(Il). It acts on bones and kidneys in concert with 1,25-dihydroxycholecalciferol to stimulate resorption of bone and reabsorption of calcium from the glomerular filtrate. Calcitonin, the third hormone, is a polypeptide secreted by the thyroid gland in response to a rise in blood Ca(Il) concentration. Its production leads to an increase in bone deposition, increased loss of calcium and phosphate in the urine, and inhibition of the synthesis of 1,25-dihydroxycholecalciferol. 

C03-0144. The thyroid gland produces hormones that help regulate body temperature, metabolic rate, reproduction, the synthesis of red blod cells, and more. Iodine must be present in the diet for these thyroid hormones to be produced. Iodine deficiency leads to sluggishness and weight gain, and can cause severe problems in the development of a fetus. One thyroid hormone is thyroxine, whose chemical formula is... 

Four small parathyroid glands are embedded on the posterior surface of the thyroid gland as it wraps around the trachea. Parathyroid hormone (PTH, parathormone) is the principal regulator of calcium metabolism. Its overall effects include ... 

Thyroid Gland A highly vascular endocrine gland consisting of two lobes, one on either side of the trachea, joined by a narrow isthmus it produces the thyroid hormones which are concerned in regulating the metabolic rate of the body. [NIH]... 

FIGURE 4.11 Negative feedback loop regulating the function of the thyroid gland. 

The thyroid hormone thyroxine (tetraiodo-thyronine, T4) and its active form triiodothyronine (T3) are derived from the amino acid tyrosine. The iodine atoms at positions 3 and 5 of the two phenol rings are characteristic of them. Post-translational synthesis of thyroxine takes place in the thyroid gland from tyrosine residues of the protein thyro-globulin, from which it is proteolytically cleaved before being released, iodothyronines are the only organic molecules in the animal organism that contain iodine. They increase the basal metabolic rate, partly by regulating mitochondrial ATP synthesis, in addition, they promote embryonic development. 

Special precautions The thyroid gland is the critical organ in terms of dose. The effective biological half-life in humans is around 140 days. Store this radioisotope behind lead shielding. Follow institutional regulations regarding (a) type and location of dosimeters and (b) monitoring thyroid for localized uptake. 

Synthesis, storage, and release of thyroid hormones by the thyroid gland are primarily regulated by the thyrotropin hormone, while the iodides necessary for their synthesis are usually present in consumed foods. 

The T4 and T3 that are released from the thyroid gland are firmly but reversibly bound to several plasma proteins. More than 99% of the circulating thyroid hormone is protein bound, with only the free hormone available to enter cells (Table 65.1).The amount of T4 or Tj entering the cells and the ultimate physiological response are directly related to the plasma concentrations of free T4 and free Tj It is the concentrations of free T4 and T3 in the plasma that are regulated by the HPTA (Fig. 65.2) rather than the total (i.e., free plus protein-bound) plasma T4 and Tj concentrations. 

The regulation of calcitonin synthesis and release from the parafollicular C cells of the thyroid gland is calcium dependent. Rising serum calcium is the principal stimulus responsible for calcitonin synthesis and release. Other hormones, such as glucagon, gastrin, and serotonin, also stimulate calcitonin release. Calcitonin has been isolated in tissues other than the parafollicular C cells (parathyroid, pancreas, thymus, adrenal), but it is not known whether this material is biologically active. 

The thyroid and parathyroid glands are two important endocrine organs that are heavily committed to the biosynthesis of hormones as chemical messengers. The thyroid gland, which surrounds the larynx, has an enormous variety of metabolic functions. It is itself regulated by thyroliberin, which in turn regulates production of thyrotropin (thyroid... 

Thyroid gland secretes two important hormones, thyroxine (TJ and triiodothyronine (Tj). The third hormone, calcitonin secreted from interstitial cells is physiologically different and is responsible for the regulation of calcium metabolism. 

The normal thyroid gland secretes sufficient amounts of the thyroid hormones—triiodothyronine (T3) and tetraiodothyronine (T4, thyroxine)—to normalize growth and development, body temperature, and energy levels. These hormones contain 59% and 65% (respectively) of iodine as an essential part of the molecule. Calcitonin, the second type of thyroid hormone, is important in the regulation of calcium metabolism and is discussed in Chapter 42. 

The thyroid gland also regulates its uptake of iodide and thyroid hormone synthesis by intrathyroidal mechanisms that are independent of TSH. These mechanisms are primarily related to the level of iodine in the blood. Large doses of iodine inhibit iodide organification (Wolff-Chaikoff block, see Figure 38-1). In certain disease states (eg, Hashimoto s thyroiditis), this can inhibit thyroid hormone synthesis and result in hypothyroidism. Hyperthyroidism can result from the loss of the Wolff-Chaikoff block in susceptible individuals (eg, multinodular goiter). 

Regulates body temperature via thyroxine Maintains thyroid gland and its secretory activity (colloid discharge)... 

Thyroid gland function also regulates calcitonin which combats elevated levels of calcium. 

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