Hypothyroidism levothyroxine

Drug-induced hypothyroidism (Table 38-3) can be satisfactorily managed with levothyroxine therapy if the offending agent cannot be stopped. In the case of amiodarone-induced hypothyroidism, levothyroxine therapy may be necessary even after discontinuance because of amiodarone s very long half-life. 

In the treatment of hypothyroidism, levothyroxine (Levo-throid and Synthroid Sodium 2 to 25 J.g/kg) is given for replacement therapy in patients with hypothyroidism. Following are other thyroid preparations ... 

Lithium is concentrated in the thyroid gland and can impair thyroid hormone synthesis. Although goiter is uncommon, as many as 30% of patients develop at least transiently elevated thyroid-stimulating hormone values. Lithium-induced hypothyroidism is not usually an indication to discontinue the drug. Patients can be supplemented with levothyroxine if continuation of lithium is desired.30... 

Describe the clinical use of levothyroxine in the treatment of hypothyroidism, including initial dose and dose titration, and patient monitoring. 

Despite the availability of a wide array of thyroid hormone products, it is clear that synthetic levothyroxine (LT4) is the treatment of choice for almost all patients with hypothyroidism. LT4 mimics the normal physiology of the thyroid gland, which secretes mostly T4 as a prohormone. As needed, based on metabolic demands, peripheral tissues convert thyroxine (T4)... 

Levothyroxine (Synthroid , Levoxyl , Unithroid ), other brands, and generics Synthetic LT4 25, 50, 75, 88, 100, 112, 125, 137, 150, 1 75, 200, and 300 meg tablets 500 meg vial for injection 60 meg Gold standard for treating hypothyroidism products not therapeutically equivalent full replacement dose 1-1.6 meg/kg per day when switching from animal product, lower calculated daily dose by 25-50 meg intravenous form rarely needed... 

Patients with subclinical hypothyroidism and marked elevations in TSH (greater than 10 miUi-intemational units per liter [mlU/L]) and high titers of TSAb or prior treatment with sodium iodide 131 may benefit from treatment with levothyroxine. 

In patients with hypothyroidism caused by hypothalamic or pituitary failure, alleviation of the clinical syndrome and restoration of serum T4 to the normal range are the only criteria available for estimating the appropriate replacement dose of levothyroxine. 

Up to 30% of patients on maintenance lithium therapy develop transiently elevated serum concentrations of thyroid-stimulating hormone, and 5% to 35% of patients develop a goiter and/or hypothyroidism, which is dose-related and more likely to occur in women. This is managed by adding levothyroxine to the regimen. 

Thyroxine (levothyroxine) is indicated in hypothyroidism as a maintenance therapy on a long-term basis. The initial dose must not exceed 100 pg. The usual maintenance dose is 100-200 pg. The dose is decreased in elderly patients. Thyroxine must be taken in the morning. 

Q77 When starting levothyroxine, a baseline ECG is recommended. The baseline ECG is used to distinguish underlying myocardial ischaemia from changes induced by hypothyroidism. 

Levothyroxine is used to treat hypothyroidism (an underactive thyroid gland). Thyroid hormone can be made from beef and pork thyroid, but this lacks standardization and it is difficult to control dosage. The synthetic drug is more desirable. Levothyroxine is one of two important thyroid hormones. It is converted into the second important hormone, liothyronine, in the body. The key step in the synthesis of structures such as levothyroxine is the substitution of an iodonium salt by an iodinated phenol. Siql-like reactions on an aromatic ring are not common, but an iodonium salt provides a good leaving group. 

Hypothyroidism (myxedema) results when there is a breakdown of thyroid hormone production in the thyroid gland. Treatment consists of replacing this hormone with aforementioned drugs. Treatment with levothyroxine, 3,5,3, 5 -tetraiodothyronine, is preferred. Lyothyronin, L-3,5,5 -triiodothyronine, is also used, as is lotrix, a mixture of levothyroxine and levothyronine in a 4 1 ratio. Of the drugs of animal origin, thyroidin and thy-roglobulin (proloid) are used. 

Levothyronine has properties of levothyroxine however, it acts faster and binds less with blood proteins. Indications for using levothyronine are the same as with levothyroxine -hypothyroidism, euthyroid goiters, thyroiditis however, its use is considered more appropriate in the first stage of treatment. Synonyms of this drug are tibon, cinomel, tertroxin, and others. 

Hypothyroidism in adults and children in whom growth and puberty are complete-The average full replacement dose of levothyroxine is approximately 1.7 mcg/kg/day (eg, 100 to 125 mcg/day for a 70 kg adult). 

Dosage adjustment The levothyroxine dose generally is adjusted in 12.5 to 25 meg increments until the patient with primary hypothyroidism is clinically euthyroid and the serum TSH has normalized. 

Severe hypothyroidism - In patients with severe hypothyroidism, the recommended initial levothyroxine dose is 12.5 to 25 mcg/day with increases of 25 mcg/day every 2 to 4 weeks. 

Subclinical hypothyroidism - If this condition is treated, a lower levothyroxine... 

Infants and children Levothyroxine therapy usually is initiated at full replacement doses, with the recommended dose per body weight decreasing with age (see table). However, in children with chronic or severe hypothyroidism, an initial 25 mcg/day dose of levothyroxine is recommended with increments of 25 meg every 2 to 4 weeks until the desired effect is achieved. 

Levothyroxine sodium (Levothwid, Synthroid, Levoxine) is the sodium salt of the naturally occurring levorota-tory isomer of T4. It is the preparation of choice for maintenance of plasma T4 and T3 concentrations for thyroid hormone replacement therapy in hypothyroid patients. It is absorbed intact from the gastrointestinal tract, and its long half-life allows for convenient once-daily administration. Since much of the T4 is deiodi-nated to T3, it is usually unnecessary to use more expensive preparations containing bothX4 and Tj.The aim is to establish euthyroidism with measured serum concentrations of T4, T3, and TSH within the normal range. 

Liotrix (Euthroid, Thywlar) is a 4 1 mixture of levothyroxine sodium and liothyronine sodium. Like levothyroxine, liotrix is used for thyroid hormone replacement therapy in hypothyroid patients. Although the idea of combining T4 and T3 in replacement therapy so as to mimic the normal ratio secreted by the thyroid gland is not new, it does not appear that liotrix offers any therapeutic advantage over levothyroxine alone. 

Levothyroxine is taken orally or intravenously and is normally prescribed to treat hypothyroidism or to suppress the release of thyroid hormone so as to manage cancerous thyroid nodules (i.e., thyroid cancer) and growth of goiters. Other less frequently prescribed thyroid and parathyroid drugs include methimazole (Tapazole), various iodides, lithioronine (Triostat), and liotrix (Thyrolar). 

Hypothyroidism can occasionally be treated with a regular intake of iodide, but in the case of non-functioning of the gland thyroxine must be used. Synthetic levothyroxine... 

Lithium Plus Thyroid Supplementation. Treatment-resistant and rapid-cycling bipolar patients may have an increased frequency of thyroid dysfunction. Further, some patients suffer from subclinical hypothyroidism and improve with the addition of thyroid supplementation. In this context, several case reports involving this population found that high doses of the thyroid hormone levothyroxine sodium (T ) were clinically beneficial (122,123 and 124). Kusalic (1.25) found that 6 of 10 rapid cyclers had hypothyroidism, based on their thyrotropin-releasing hormone stimulation tests. Further, the average number of mood episodes per year decreased by more than 75% (i.e., from 9.7 to 2.2) after thyroxine was added to the treatment regimen. 

Levothyroxine (T4 ) Activation of nuclear receptors results in gene expression with RNA formation and protein synthesis Hypothyroidism See Table 38-1 maximum effect seen after 6-8 weeks of therapy Toxicity See Table 38-4 for symptoms of thyroid excess... 

Dong BJ et al Bioequivalence of generic and brand-name levothyroxine products in the treatment of hypothyroidism. JAMA 1997 277 1205. [PMID 9103344]... 

Neonatal goiter caused by the use of potassium iodine as an expectorant during pregnancy has been reported (67). The neonate, a girl, had acute hypothyroidism, with myxedema and respiratory distress. She was given levothyroxine for 6 months, with complete normalization of thyroid function. 

Both the thiouracils and thioimidazoles readily cross the placenta and can cause fetal hypothyroidism, resulting in a slight delay in neurological or bone maturation. Various degrees of goiter have also been observed, even to the extent of severe tracheal compression and death. Antithyroid drug dosage should therefore be reduced to the minimum required to maintain a euthyroid state without supplementation of levothyroxine (93). 

Levothyroxine is used as replacement therapy in hypothyroidism and to suppress the production of thyro-trophin (thyroid-stimulating hormone) in patients with thyroid carcinoma. 

As with all forms of long-term therapy, adherence to the prescribed dosage of levothyroxine is not always optimal, and an unwarranted fear of thyroid-induced osteoporosis can add to this lack of adherence. Inadequacy of thyroxine replacement therapy is not always easily recognized. Several patients were reported with clearly inadequate or excessive consumption of levothyroxine despite a correct prescription. All patients had depression, which could be an additional susceptibility factor by promoting lack of adherence, and the resulting hypothyroidism or hyperthyroidism could further aggravate the depression (12). 

When replacement therapy is with levothyroxine only, the T4/T3 ratio is increased compared with healthy subjects, suggesting that thyroid secretion of T3 is physiologically important. Animal studies have shown that euthyroidism is not restored in all tissues by levothyroxine alone (13). Mood and neuropsychological function improved in hypothyroid patients when 50 micrograms of thyroxine was replaced by 12.5 micrograms of liothyr-onine (14). 

A 71-year-old woman who had undergone total thyroidectomy with subsequent irradiation because of follicular carcinoma 3 years before (22). Since then, she had taken oral levothyroxine 0.15 mg and 0.2 mg on alternate days. When latent hypothyroidism became evident despite replacement therapy, the dose of levothyroxine was increased to 0.3 mg/day. Three weeks later, she had formed an acute posterior myocardial infarction, although she had no previous history of coronary artery disease. Subsequent coronary arteriograms revealed no evidence of disease of the major vessels. Myocardial scintigraphy 3 weeks after infarction still showed a persistent perfusion defect. 

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