部分中文左甲状腺素处方资料（仅供参考）
药品英文名
Levothyroxine
药品别名
左甲状腺素、爱初新、甲四碘妥、左甲状腺素钠、特洛新、优甲乐、L-T4、Eltroxin、Euthyrox、Levaxin、Thyxine Sodium、Levothyroxine Sodium
药物剂型
片剂：50μg，100μg。
药理作用
人体血液甲状腺素（T3、T4)中T4占90%，T3的量虽少，但生物活性强，本品在体内转变成三碘甲腺氨酸（T3)，对甲状腺功能减退症产生治疗作用。本品可通过人体内分泌的反馈作用机制，抑制甲状腺素（TSH)的分泌，减少甲状腺TSH受体和血液中的TSH受体拮抗，预防甲亢复发。用时抑制脑垂体TSH的过度分泌，从而有效的预防甲状腺肿复发。
药动学
口服易吸收，经用131I标记的甲状腺素研究证明，血浆半衰期为7～8天，组织中为0.4天。吸收后，大部分与血浆蛋白牢固结合而徐徐进入组织细胞中，血浆蛋白结合率为99%，但T3对血浆蛋白的亲和力比T4低，其游离量为后者的10倍。其最大治疗效果经数周后方可达到，调整剂量反应慢。停药后亦可维持药效数周。T4主要在肝、肾线粒体内脱碘，并与葡萄糖醛酸或硫酸结合经肾脏由尿液中排出。
适应证
用于甲状腺功能减退症。也可用于甲状腺滤泡细胞癌和甲状腺乳头状癌的辅助治疗以及用于甲状腺碘抑制试验。
禁忌证
多种原因所致的甲状腺亢进或甲状腺毒症者禁用。
注意事项
1.老年人慎用。
2.妊娠妇女慎用。
不良反应
剂量较大时可出现心绞痛、心律失常、心悸、骨骼肌痉挛、心动过速、腹泻、烦躁不安、兴奋、失眠、头痛、潮红、出汗、体重过量减轻等，但当降低剂量或停药数日后会逐渐消失。
用法用量
口服：
1.成人：初始剂量每天50～100μg，每天1次，随后每隔3～4周以50μg调整至适宜剂量，以保证稳定的正常新陈代谢，这可能需要每天200～300μg或较大剂量。若增加新陈代谢的功能产生过速（导致腹泻、神经过敏、脉搏加快、失眠、震颤及在有潜伏性的心肌局部缺血情况时会心绞痛)，应将剂量减低或停止1～2天，然后再从较低剂量开始。有冠心病或其他心血管病存在时，应用25μg为首剂量较合适，隔4周后再每天增加25μg。
2.儿童：剂量为0～6个月儿童，每天25～50μg，6～12个月每天50～75μg，1～5岁每天75～100μg，6～12岁每天100～150μg。3.用于呆小病及幼年黏液性水肿，应用最大而又不致中毒的剂量（脉搏正常，无腹泻或便秘)。一般初始剂量为每天25μg，每2～4周增加25μg，直至出现轻度中毒症状，然后稍微减量维持治疗。幼年黏液性水肿症初始剂量每天50～100μg，维持剂量第1年通常为每天100μg，较后幼年期升至300μg或较大剂量。空腹吸收最好，一般早餐前服用。用于甲状腺碘抑制试验，每天26μg/kg，连续7天。
NDC: 00378-1819-77
Levothyroxine Sodium
Pharmacological Classifications: Thyroid Prohormone (hormone precursor)
General Information:
Levothyroxine (T4) is a synthetically prepared levo-isomer of thyroxine, a hormone secreted by the thyroid gland. Levothyroxine is used in the treatment of primary, secondary (pituitary), and tertiary (hypothalamic) hypothyroidism.
Levothyroxine will potently suppress thyrotropin secretion in the management of goiter and chronic lymphocytic thyroiditis, and it can be used in combination with antithyroid agents to prevent the abc development of hypothyroidism or goitrogenesis during the treatment of thyrotoxicosis. Intravenous levothyroxine is primarily used to treat myxedema coma or stupor.
Levothyroxine therapy is preferred over thyroid and thyroglobulin because the hormonal content of levothyroxine is standardized, and the effects of the drug are more predictable. Levothyroxine provides only T4, of which roughly 80% is deiodinated to T3 and reverse T3.
Since T3 is three times as potent as T4, virtually all of the activity of T4 can be ascribed to T3.
Levothyroxine has been used clinically since the 1950s. Thyroid drugs containing levothyroxine sodium were sold for years without FDA approval. For many years, there has been controversy regarding the bioequivalence of different oral levothyroxine products, which had not been reviewed by modern FDA approval processes.
A controversial study published in Gericke KR. Possible interaction between warfarin and fluconazole. Pharmacotherapy 1993;13:508—9.7 showed that several products were bioequivalent.
Subsequently, in August Gericke KR. Possible interaction between warfarin and fluconazole. Pharmacotherapy 1993;13:508—9.7, the FDA announced that NDAs would be required for all oral levothyroxine products by August 2003. The required NDA will ensure that FDA standards for purity, potency, stability, safety and efficacy are met. Unithroid® and Levoxyl® received an FDA approval via the new NDA requirements in August 2000 and May 2001, respectively.
Levo-T® received formal FDA-approval in early 2002; followed by the FDA-approval of Synthroid®, after 47 years of marketing, in July 2002.
Levolet® products were FDA-approved in 2003; Levothroid® products were FDA-approved in 2004. Tirosint™ capsules were FDA-approved in October of 2006. The NDA status of other marketed brands is not clear.
Mechanism of Action: Levothyroxine exhibits all the actions of endogenous thyroid hormone.
Liothyronine (T3) is the principal hormone that exhibits these actions whereas levothyroxine (T4) is the major hormone secreted by the thyroid gland and is metabolically deiodinated to T3 in peripheral tissues. Serum concentrations of T4 and TSH are typically used as the primary monitoring parameters for determining thyroid function.
In general, thyroid hormones influence the growth and maturation of tissues, increase energy expenditure, and affect the turnover of essentially all substrates. These effects are mediated through control of DNA transcription and, ultimately, protein synthesis.
Thyroid hormones play an integral role in both anabolic and catabolic processes and are particularly important to the development of the central nervous system in newborns.
They regulate cell differentiation and proliferation, and aid in the myelination of nerves and the development of axonal and dendritic processes in the nervous system. Thyroid hormones, along with somatotropin, are responsible for regulating growth, particularly of bones and teeth.
Thyroid hormones also decrease cholesterol concentrations in the liver and the bloodstream, and have a direct cardiostimulatory action.
Cardiac consumption is increased by the administration of thyroid hormone, resulting in an increased cardiac output. Administration of exogenous thyroid hormone to patients with hypothyroidism increases the metabolic rate by enhancing protein and carbohydrate metabolism, increasing gluconeogenesis, facilitating the mobilization of glycogen stores, and increasing protein synthesis. In response to reestablishing physiologic levels of thyroid hormone, thyroid-stimulating hormone (TSH) concentrations correct if the primary disorder is at the level of the thyroid.
The release of T3 and T4 from the thyroid gland into the systemic circulation is regulated by TSH (thyrotropin), which is secreted by the anterior pituitary gland. Thyrotropin release is controlled by the secretion of thyroid-releasing hormone (TRH) from the hypothalamus and by a feedback mechanism dependent on the concentrations of circulating thyroid hormones. When circulating T3 and T4 levels increase, TRH and TSH decrease. Because of this feedback mechanism, the administration of pharmacologic doses of exogenous thyroid hormone to patients with a normal thyroid suppresses endogenous thyroid hormone secretion.
Pharmacokinetics: Levothyroxine is administered orally or intravenously. Over 99% of levothyroxine (T4) is bound to proteins, primarily thyroxine-binding globulin