MSEM101 PDF - Anatomic and Physiologic Overview

11/20/23, 12:58 AM Realizeit for Student Anatomic and Physiologic Overview Various hormones and chemicals are responsible for normal thyroid function. Key among them are thyroid hormone, calcitonin, and iodine. Thyroid Hormone Thyroid hormone is comprised of T4 and T3, two separate hormones produc...

11/20/23, 12:58 AM Realizeit for Student Anatomic and Physiologic Overview Various hormones and chemicals are responsible for normal thyroid function. Key among them are thyroid hormone, calcitonin, and iodine. Thyroid Hormone Thyroid hormone is comprised of T4 and T3, two separate hormones produced by the thyroid gland. Both are amino acids that contain iodine molecules bound to the amino acid structure; T4 contains four iodine atoms in each molecule, and T3 contains three. These hormones are synthesized and stored bound to proteins in the cells of the thyroid gland until needed for release into the bloodstream. Three thyroid-binding hormones— thyroxine-binding globulin (TBG), transthyretin (formerly known as thyroid-binding prealbumin), and albumin—bind and transport T3 and T4 (Norris, 2019). Synthesis of Thyroid Hormone https://herzing.realizeithome.com/RealizeitApp/Student.aspx?Token=0Dn26kXyU%2f6F5gOCz4%2f2IZN5n24G8UktHULazYLeTIBFBizuuh93swtew1H3xVYA2sj… 1/6 11/20/23, 12:58 AM Realizeit for Student Iodine is essential to the thyroid gland for synthesis of its hormones. The major use of iodine in the body is by the thyroid, and the major derangement in iodine deficiency is alteration of thyroid function. Iodide is ingested in the diet and absorbed into the blood in the GI tract. The thyroid gland is extremely efficient at taking up iodide from the blood and concentrating it within the cells, where iodide ions are converted to iodine molecules, which react with tyrosine (an amino acid) to form the thyroid hormones (Norris, 2019). Regulation of Thyroid Hormone The secretion of T3 and T4 by the thyroid gland is controlled by TSH (also called thyrotropin) from the anterior pituitary gland. TSH controls the rate of thyroid hormone release through a negative feedback mechanism. In turn, the level of thyroid hormone in the blood determines the release of TSH. If the thyroid hormone concentration in the blood decreases, the release of TSH increases, which causes increased output of T3 and T4. The term euthyroid refers to thyroid hormone production that is normal. TRH, secreted by the hypothalamus, exerts a modulating influence on the release of TSH from the pituitary. Environmental factors, such as a decrease in temperature, may lead to increased secretion of TRH, resulting in elevated secretion of thyroid hormones. Figure 45-4 shows the hypothalamic–pituitary–thyroid axis, which regulates thyroid hormone production. https://herzing.realizeithome.com/RealizeitApp/Student.aspx?Token=0Dn26kXyU%2f6F5gOCz4%2f2IZN5n24G8UktHULazYLeTIBFBizuuh93swtew1H3xVYA2sj… 2/6 11/20/23, 12:58 AM Realizeit for Student Function of Thyroid Hormone The main function of thyroid hormone is to control cellular metabolic activity. T4, a relatively weak hormone, maintains body metabolism in a steady state. T3 is about five times as potent as T4 and has a more rapid metabolic action. These hormones accelerate metabolic processes by increasing the level of specific enzymes that contribute to oxygen consumption and altering the responsiveness of tissues to other hormones. The thyroid hormones influence cell replication, are important in brain development, and are necessary for normal growth. Thyroid hormones affect virtually https://herzing.realizeithome.com/RealizeitApp/Student.aspx?Token=0Dn26kXyU%2f6F5gOCz4%2f2IZN5n24G8UktHULazYLeTIBFBizuuh93swtew1H3xVYA2sj… 3/6 11/20/23, 12:58 AM Realizeit for Student every major organ system and tissue function, including the basal metabolic rate, tissue thermogenesis, serum cholesterol levels, and vascular resistance (Norris, 2019). Calcitonin Calcitonin, or thyrocalcitonin, is another important hormone secreted by the thyroid gland. The hormone is secreted in response to high plasma levels of calcium, and it reduces the plasma level of calcium by increasing its deposition in bone. Pathophysiology Congenital hypothyroidism, which occurs when there is inadequate secretion of thyroid hormone during fetal and neonatal development, results in intellectual disability and stunted physical growth because of general depression of metabolic activity (Norris, 2019). In adults, hypothyroidism manifests as lethargy, slow mentation, weight gain, constipation, cold intolerance, and generalized slowing of body functions (Singh & Clutter, 2019). Hyperthyroidism (oversecretion of thyroid hormones) is manifested by a greatly increased metabolic rate. Many of the other characteristics of hyperthyroidism result from the increased response to circulating catecholamines (epinephrine and norepinephrine). Oversecretion of thyroid hormones is usually associated with an enlarged thyroid gland known as a goiter. Goiter also commonly occurs with iodine deficiency. In this latter condition, lack of iodine results in low levels of circulating thyroid hormones, which causes increased release of TSH; the elevated TSH causes overproduction of thyroglobulin (a precursor of T3 and T4) and hypertrophy of the thyroid gland. Assessment Diagnostic Evaluation Assessment measures in addition to palpation and auscultation include thyroid function tests, such as laboratory measurement of thyroid hormones, thyroid scanning, biopsy, and ultrasonography. The most widely used tests are serum immunoassay for TSH and free T4. Free T4 levels correlate with metabolic status; they are elevated in hyperthyroidism and decreased in hypothyroidism (Fischbach & Fischbach, 2018). Ultrasound, CT, and MRI may be used to clarify or confirm the results of other diagnostic studies. https://herzing.realizeithome.com/RealizeitApp/Student.aspx?Token=0Dn26kXyU%2f6F5gOCz4%2f2IZN5n24G8UktHULazYLeTIBFBizuuh93swtew1H3xVYA2sj… 4/6 11/20/23, 12:58 AM Realizeit for Student Nursing Implications Since thyroid tests involve the use of iodine, determining if the patient has any allergies to iodine or is taking medications that contain iodine is essential. The relationship between having an allergy to shellfish and having an allergy to iodine is a long held belief; however, an allergy to shellfish is due to specific proteins in the shellfish and not iodine (American College of Allergy, Asthma and Immunology, 2019). Patients should be asked if they have had a reaction to iodine previously and to shellfish so that the radiologist can determine what precautions need to be taken, if any (American College of Allergy, Asthma and Immunology, 2019). Patients should be asked about obvious sources of iodine-containing medications such as contrast agents and those used to treat thyroid disorders such as radioactive iodine. They should also be asked whether they eat kelp or seaweed. Numerous medications may also affect test results because they affect the thyroid. Chart 45-2 gives a list of select medications that may interfere with accurate testing of thyroid gland function (Fischbach & Fischbach, 2018). This information should be documented in the patient’s electronic health record (EHR) and communicated clearly to personnel conducting the test. Thyroid Tests Serum Thyroid-Stimulating Hormone Measurement of the serum TSH concentration is the primary screening test of thyroid function. The ability to detect minute changes in serum TSH makes it possible to distinguish subclinical thyroid disease from euthyroid states in patients with low or high normal values. Measurement of TSH is also used for monitoring thyroid hormone replacement therapy and for differentiating between disorders of the thyroid gland itself and disorders of the pituitary or hypothalamus. https://herzing.realizeithome.com/RealizeitApp/Student.aspx?Token=0Dn26kXyU%2f6F5gOCz4%2f2IZN5n24G8UktHULazYLeTIBFBizuuh93swtew1H3xVYA2sj… 5/6 11/20/23, 12:58 AM Realizeit for Student The American Thyroid Association recommends that pregnant women be screened for thyroid disease (Alexander, Pearce, Brent, et al., 2017); however, there is lack of consensus among professional organizations regarding routine screening of adults for thyroid disease. The US Preventive Services Task Force (USPSTF) does not recommend routine screening of adults (USPSTF, 2019). Serum Free T4 Serum free T4 is a direct measurement of free (unbound) thyroxine, the only metabolically active fraction of T4. The range of free T4 in serum is normally 0.7 to 2.0 ng/dL (10 to 26 pmol/L) (Fischbach & Fischbach, 2018). When measured by the dialysis method, free T4 is not affected by variations in protein binding and is the procedure of choice for monitoring the changes in T4 secretion during treatment for hyperthyroidism. Serum T3 and T4 Measurement of total T3 or T4 includes protein-bound and free hormone levels that occur in response to TSH secretion. T4 is 70% bound to TBG; T3 is bound less firmly. Only 0.03% of T4 and 0.3% of T3 are unbound. Serious systemic illnesses, medications (e.g., oral contraceptives, corticosteroids, carbamazepine, salicylates), and protein wasting as a result of nephrosis or the use of androgens may interfere with accurate test results. Normal range for T4 is 5.4 to 11.5 µg/dL (57 to 148 nmol/L) (Fischbach & Fischbach, 2018). Although serum T3 and T4 levels generally increase or decrease together, the T3 level appears to be a more accurate indicator of hyperthyroidism or severity of the disorder, as T4 levels are often within normal range. The normal range for serum T3 is 260 to 480 pg/dL (4.0 to 7.4 pmol/L) (Fischbach & Fischbach, 2018). https://herzing.realizeithome.com/RealizeitApp/Student.aspx?Token=0Dn26kXyU%2f6F5gOCz4%2f2IZN5n24G8UktHULazYLeTIBFBizuuh93swtew1H3xVYA2sj… 6/6