甲状腺機能低下症の病態生理:ヨード代謝を中心にして
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概要
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It has been well known that hypothyroidism is caused by hypofunction of thyroid gland, in resulting in decrease of the level of circulating thyroid hormone. Since the function of thyroid gland is regulated by thyroid stimulating hormone (TSH) secreted from pituitary gland, hypofunction of the pituitary gland produces the hypothyroidism known well as secondary myxedema. In this type of myxedema, the production of thyroid hormone can be reduced by a defect in at least three steps of hormones biosynthesis, especially in term of thyroidal iodine metabolism as follow : (1) decrease of thyroidal radioiodine uptake (2) reduction of formation of both thyroxine (T4) and triiodothyronine (T3) (3) slowering the release of thyroid hormone from the gland into the blood. As well known, the facts are resulted by the reduction of TSH secretion from the pituitary gland. However, TSH level in the blood of primary myxedema is usually higher than that of secondary hypothyroidism. For examples, so-called idiopathic myxedema might have almost the same defect of thyroidal hormone synthetic step as that in the secondary hypothyroidism but TSH level in such patients can be over normal range due to negative feedback mechanism. Clinical pictures of patients with chronic thyroiditis suggested the presence of hypothyroidism but thyroidal radioiodine uptake of some of such patients showed within normal range because of over stimulation of thyroid by TSH. Since it has also well known that most of the primary hypothyroidism in the adult are followed by chronic thyroiditis, recently we studied the iodine metabolism in the thyroid of the primary myxedema and it was found that the myxedema could be caused by not only the atrophy of thyroid gland but also a defect of oxidation of iodide in the thyroid of chronic thyroiditis. The present report deals with our findings concerning abnormal radioiodine metabolism with mainly chromic thyroiditis.<BR>Goiter KSCN-Test After I-Loading KSCN-Test BMR Uptake Goiter Idiopatic myxedema - np np dec dec - Myxedema due to chronic throdidstis + + np dec dec e radioiodine + - + dec dec c Masked hypothyroidism (A) - +or- + dec dec - (B) + - + dec dec c (c) + - - inc dec dec np indicates not performed ; - indicates no goiter or goiter was not present : + indicates presence of goiter : c indicates constant size of goiter : dec indicates decrease : inc indicates increase.<BR>Clinical thyroid function's tests such as basal metabolic test (BMR), protein bound iodine in the serum (PBI), resin sponge T3 uptake (RSU) and thyroidal radioiodine 24 hr uptake (Uptake) were measured in all of patients who showed typical or subclinical signs of hypothyroidism. Then, thiocyanate test was employed to all patients with uptake more than 10% to know whether or not there might be present a defect of oxidation of iodide in the thyroid. Thereafter, iodinated lecithin containing 1 mg iodide administered to patients once daily for 14 days and again BMR, PBI, RSU and Uptake were measured (Iodine loading test). In a patient with negative thiocyanate test before iodine loading, this test was again performed after iodine loading. Primary myxedema except due to antithyroid drugs was divided into several groups according to both thiocyanate test and iodine loading test.<BR>Clinical pictures suggested the presence of hypothyroidism but thyroid function's tests showed within normal range, in which some type of abnormal thyroidal radioiodine metabolism could be found by applying the thiocyanate and iodine loading tests. We called such patients as masked hypothyroidism.
- 日本内分泌学会の論文