A. The Thyroid Gland and Its Hormones

 The thyroid gland is the largest of the endocrine organs and stores more of its hormone than any of the other endocrine glands. Through the secretion of thyroxine, T4, and its active derivative, T3, the thyroid governs myriad physiological events which bring about functional alterations in virtually all metabolic pathways and organs. Thyroid hormone modulates oxygen consumption and basal metabolic rate (BMR), as well as lipid, carbohydrate, and protein metabolism. It has fundamental effects on the nervous system both during development and in childhood and adulthood. Thyroid hormone regulates the synthesis and degradation rates of numerous proteins, including other growth factors and hormones, so that many of its effects emerge as secondary influences on other endocrine pathways.

The effects of thyroid hormone fall into two categories of biological responses: (a) effects on cellular differentiation and development, particularly, but not limited to, the nervous system; and (b) effects on metabolic pathways through which the body uses carbohydrates, lipids, and proteins. These two actions are interconnected in that changes in development and growth are both a cause and a consequence of hormonal modulation of metabolism. The wide range of the biological effects of thyroid hormone is reflected in the fact that virtually every tissue in the body expresses a receptor, TR, for the active hormone, T3. The presence of different forms of the receptor, TRα or TRβ, in different tissues expands the range of effects this hormone can have in its target tissues.

With an understanding of thyroid hormone secretion, its regulation, and its actions and some of their mechanisms, the stage will be set to observe the results of excess or insufficiency of this hormone. The symptoms of the most common disease states of the thyroid gland are a direct consequence of an interruption of the known actions of its hormones. Furthermore, treatments of these conditions are based directly on our understanding of the mechanisms of the synthesis of thyroid hormone and its regulation.

B. Iodine Metabolism

The principal iodinated products produced by the thy roid gland are thyroxine (T4) and triiodothyronine (T3), which contain four and three atoms of organically bound iodine, respectively. Accordingly, the normal functioning of the thyroid is dependent on adequate and regular dietary intake of iodine.

Iodine is a rare element; although present in ocean water, it is distributed unevenly in the soils of the various land masses of the world and therefore the food content of iodine varies widely throughout the world. It is estimated that about 2 billion people suffer from iodine deficiency, which is the leading preventable cause of mental retardation in the world. The recommended daily allowance (RDA) of iodine established by the U.S. National Research Council is 150 μg/day for adults. In the United States, the average daily intake of iodine is in the range of 250–700 μg/day, due largely to iodized salt and iodate in bread. The metabolic fates of this iodine intake are depicted in Figure 1.

Fig1. Physiology and distribution of iodide. In a healthy adult with a dietary intake of 400 μg iodine (I) per day, approximately 115 μg is accumulated across the thyroid epithelial cell membrane. Of this amount 75 μg is incorporated into thyroid hormones. ECF = extracellular fluid (25 liters). Numbers in parentheses indicate the three major pools of I−: the blood pool as circulating T3 or T4; the ECF pool as free I−, and the thyroid gland pool as protein-bound or free I−.

In the absence of adequate dietary access to iodine, an individual will adaptively develop iodine deficiency; endemic goiter exists when more than 10% of preadolescent children in a population grouping have enlarged, mildly hypertrophied thyroid glands due to iodine deficiency. In most adults with endemic goiter, iodine intake is below 50 μg/day. Endemic goiter does not exist in the United States, but can be found in areas of the world in which there is a suboptimal level of iodine in the soil and in food crops grown thereon, e.g., large areas of central Africa, central Asia, the Andes of South America, and Indonesia. Endemic goiter can be prevented by various interventions, including the oral administration of potassium iodide at 6-month intervals.

اخر الاخبار

اخبار العتبة العباسية المقدسة

خدمات صحية متكاملة قدَّمها قسم الشؤون الطبية لزائري ليلة النصف من شعبان في مجمَّع الشيخ الكليني

خدمات صحية متكاملة قدَّمها قسم الشؤون الطبية لزائري ليلة النصف من شعبان في مجمَّع الشيخ الكليني

في ذكرى ولادة الإمام المهدي (عجّل الله فرجه).. قسم الشؤون الفكرية يصدر عددًا خاصًّا من مجلة العقيدة

للاطّلاع على خدماته المقدمة للزائرين مركز السيد جعفر الحلي يستقبل المدير العام لمديرية العمليات والخدمات الطبية في وزارة الصحة

المزيد

الآخبار الصحية

دراسة تكشف "سر الوسادة".. ودورها في حماية البصر

بالأشعة تحت الحمراء.. "ثورة علمية" في علاج اللاعبين

مشروب أحمر يخفض ضغط الدم "بشكل ملحوظ"

الصحة العالمية تكشف حقيقة انتشار فيروس نيباه خارج الهند

المزيد

الآخبار التكنلوجية

دراسة تدفعك لشراء ساعة ذكية "فورا".. هذا ما تقوله عن قلبك

علماء حاصلون على "نوبل": الذكاء الاصطناعي لن يستبدل الإنسان

وانغ هو: القمة العالمية للعلماء تجسد تقدير الإمارات للعلم

رئيس رابطة كبار العلماء يعلن تأسيس مقر في الإمارات

المزيد

مواضيع ذات صلة

Thyroglobulin Storage, Endocytosis, and Breakdown

Thyroid Peroxidase and DUOX: Tyrosine Iodination and Coupling

Thyroglobulin

The Thyroid Epithelial Cell

 Regulation of Calcemia and Calcium- Phosphorus Homeostasis

Biological Actions of Parathormone (PTH)

Synthesis, Secretion, Metabolism, and Mechanism of Action of Parathormone (PTH)

Regulation of Thyroid Hormone Secretion and TSH

Biosynthesis and Secretion of Thyroid Hormones

العمل الجزيئي لهرمون الغدة الدرقية

الغدة الدرقية Thyroid gland

الغدة جار الدرقية The Parathyroid gland