Porth's Essentials of Pathophysiology, 4e

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Mechanisms of Endocrine Control

C h a p t e r 3 1

The cloning of many endocrine system genes has had an enormous impact on everyday clinical practice. For example, identification of a gene for a given disorder (e.g., the RET proto-oncogene in certain multiple endo- crine neoplasia syndromes) means that faster diagno- sis and more appropriate management for the affected individual can occur, but also that screening of family members for kindred harboring a known mutation can be undertaken. ImagingTechniques Imaging studies are important in the diagnosis and follow-up of endocrine disorders. Imaging modalities related to endocrinology can be divided into isotopic and nonisotopic types. Isotopic imaging includes radioactive scanning of the thyroid (e.g., using radioiodine) and parathyroids (e.g., using sestamibi). Nonisotopic imag- ing includes magnetic resonance imaging (MRI), which is the preferred choice for pituitary and hypothalamic imaging, and computed tomography (CT) scanning, which is preferred for adrenal lesions and abdomi- nal endocrine lesions. Ultrasonographic scanning pro- vides excellent and reproducible anatomic images for the thyroid, parathyroids, and neighboring structures. Thyroid ultrasonography is recommended for managing thyroid nodules and can aid in visualization of the nod- ule for biopsy (fine needle aspiration), which is neces- sary to help distinguish benign from malignant etiology. Selective venography is usually accompanied by venous sampling to determine hormonal output from a gland or organ (e.g., adrenal, pituitary, and kidney). Positron emission tomography (PET) scanning is being used more widely for evaluation of endocrine tumors. Dual electron x-ray absorptiometry (DEXA) is used routinely for the diagnosis and monitoring of osteoporosis and metabolic bone diseases. ■■ The endocrine system acts as a communication system that uses hormones as chemical messengers. Hormones travel through the blood to distant target sites of action, or act locally as paracrine or autocrine messengers to incite more local effects. ■■ Hormones exert their effects by interacting with high-affinity receptors, which in turn are linked to one or more effector systems in the cell. Some hormone receptors are located on the surface of the cell and act through second messenger mechanisms, and others are located in the cell, where they modulate the synthesis of enzymes, transport proteins, or structural proteins. Other hormones bind to nuclear receptors and act by directly regulating gene transcription. SUMMARY CONCEPTS

R E V I EW E X E R C I S E S 1. Vitamin D is often considered a hormone rather than a vitamin. A. Explain. 2. Thyroid hormones are transported in the serum bound to transport proteins such as thyroid- binding globulin and albumin. A. Explain why free thyroxine (T 4 ) levels are usually used to assess thyroid function rather than total T 4 levels. 3. People who are being treated with exogenous forms of corticosteroid hormones often experience diminished levels of ACTH and exogenously produced cortisol. A. Explain using information regarding the hypothalamic-pituitary feedback control of cortisol production by the adrenal cortex. ■■ Many of the endrocrine glands are under the regulatory control of other parts of the endocrine and nervous systems.The hypothalamus and the pituitary gland form a complex integrative network that joins the nervous system and the endocrine system; this central network controls the output from many of the other glands in the body. ■■ Many hormones are controlled by negative feedback loops that adjust their level and confer stability to the many functions of the endocrine system. ■■ Endocrine function can be assessed directly by measuring blood hormone levels or indirectly by assessing the effects that a hormone has on the body (e.g., assessment of insulin function through blood glucose). Imaging techniques are increasingly used to visualize endocrine structures, and genetic techniques are used to determine the presence of genes that contribute to the development of endocrine disorders.

B I B L I O G R A P H Y

Alberts B, Aranda A, Pascual A. Nuclear hormone receptors and gene expression. Physiol Rev. 2001;81(3):1239–1304. Alberts B, Johnson A, Lewis V, et al. Molecular Biology of the Cell . 5th ed. New York, NY: Garland Science; 2008:889–903.