![]() ![]() The term cretinism, used to define the severe impairment of physical and neurological development resulting from iodine deficiency during fetal and post-natal development, derived from ‘cretein’, a term first used in the Swiss Alps. Areas of iodine deficiency tend to be inland, at high altitude and isolated and daily iodine intake may be as low as 25 μg. Fruit and vegetables also contain significant concentrations of iodine, although the amount depends on the soil and growing region. Iodine (as the iodide, I -) is relatively abundant in seawater and seafood is a rich dietary source. In contrast, lack of iodine in the diet leads to hypothyroidism. The patient in Clinical Case 3.1 was, thus, taking over 500 times the normal intake (released upon the breakdown of amiodarone) resulting in an excess production and release of thyroid hormones and symptoms of hyperthyroidism. The normal European daily dietary intake of iodine is about 150 μg, of which approximately 125 μg is taken up by the thyroid gland and used for hormone synthesis. Clinical examination revealed a diffusely enlarged thyroid gland ( Box 3.4) and blood tests later confirmed that she was hyperthyroid. An electrocardiogram confirmed the irregular tachycardia and a diagnosis of a left cerebral hemisphere stroke (causing the expressive dysphasia), secondary to atrial fibrillation, was made. She had attributed recent anxiety, poor sleeping patterns, intolerance of summer weather and a 3 kg weight loss to the worry over the job interview. By writing, she was able to inform the medical team that she was taking the drug amiodarone for a heart condition that runs in her father's family. She was found to have an irregular pulse rate of 180 beats per minute and remained unable to find her words (termed an expressive dysphasia). The sympathetic secretary in the outer office ordered her a taxi to the hospital. During the interview she suddenly found herself unable to speak and left the room unsteadily and in tears under the gaze of her disbelieving interviewers. Clinical Case 3.1 illustrates the importance of iodine in thyroid function and, whilst it is relatively unusual, it demonstrates the problems of excess iodine intake and a consequent increase in thyroid hormone synthesis and release.Ī 25-year-old city high-flyer was in an interview for a position with a merchant bank. The thyroid gland has evolved not only to trap this element avidly from dietary sources but also to maintain a large store of the iodinated tyrosines to maintain the secretion of thyroid hormones during periods of relative iodine deficiency. Whilst tyrosine is relatively easily iodinated, iodine is rare, ranking 61st in the list of most common elements and forming just 0.000006% of the Earth's mantle. The synthesis of thyroid hormones requires the iodination of tyrosine molecules and the combination of two iodinated tyrosine residues ( Box 3.1). Like the catecholamines epinephrine and norepinephrine, thyroid hormones are synthesized from the amino acid tyrosine. Disorders associated with altered thyroid hormone secretion are common and affect about 5% women and 0.5% men. They act on virtually every cell in the body to alter gene transcription: under- or over-production of these hormones has potent effects. Thyroid hormones are extremely important and have diverse actions. King Henry VI Part III, William Shakespeare. Investigation and treatment of thyroid diseaseīlown with the windy tempest of my heart.’ Physiological control of thyroid growth and hormone secretion 5.Ĭauses and clinical effects of insufficient and excessive thyroid hormone secretion 6. Physiological roles of thyroxine and tri-iodothyronine, their transport and metabolism 4. Mechanisms of iodination of tyrosine and their regulation 3. Importance of iodine in thyroid metabolism 2. ![]()
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