Levothyroxine dosing

Maintenance levothyroxine doses are typically 75 to 100 meg IV until the patient stabilizes and oral therapy is begun. 

Serum TSH concentration is the most sensitive and specific monitoring parameter for adjustment of levothyroxine dose. Concentrations begin to fall within hours and are usually normalized within 2 to 6 weeks. 

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. 

In the treatment of well-differentiated (papillary and follicular) thyroid cancer, levothyroxine is used as an adjunct to surgery and radioiodine therapy. Generally, TSH is suppressed to less than 0.1 milliunits/L, and this usually requires a levothyroxine dose of greater than 2 mcg/kg/day. However, in patients with high-risk tumors, the target level for TSH suppression may be less than 0.01 milliunits/L. 

The frequency of TSH monitoring during levothyroxine dose titration is generally recommended at 6- to 8-week intervals until normalization. For patients who have recently initiated levothyroxine therapy and whose serum TSH has normalized or in patients who have had their dosage or brand of levothyroxine changed, measure the serum TSH concentration after 8 to 12 weeks. 

THYROID HORMONES H2 RECEPTOR BLOCKERS-CIMETIDINE 1 efficacy of levothyroxine 1 absorption Clinical significance unclear. Monitor requirement for t levothyroxine dose... 

Despite the wide adoption of aut idem regulations in many countries, levothyroxine preparations are not readily interchangeable, because the galenic properties of generic brands have been shown to vary considerably and exert variable degrees of TSH suppression at the same levothyroxine dose (Krehan etat, 2002). 

Factors that affect control of thyroid function in elderly persons include noncompliance with therapy, variability in absorption, variation in the biological activity of levothyroxine tablets, or the influence of drugs on TSH levels. Rapid changes in levothyroxine dose may also cause delays in the control of patients. The half life of T4 is about 1 week, but is longer in patients with hypothyroidism. This is the reason why a lapse of 3—6 weeks would be needed forT4 concentrations to reach a steady state and to evaluate the need for a change in dose. 

Notes-. Levothyroxine doses and percentages iisted were taken (or recaicuiated) from the studies shown as references. Levothyroxine doses are expressed as the mean doses. More of the hypothyroid patients receiving ievothyroxine before pregnancy need to increase ievothyroxine dose by between 20% and 50% according to the etioiogy of the hypothyroidism. L-T4, ievothyroxine HASH, Hashimoto s thyroiditis ABL, thyroid abiation NS, nonspecified. Reproduced with modifications from Mandei, (2004) with permission from Eisevier. 

An 80-year-old patient with advanced thyroid cancer taking levothyroxine 125 micrograms daily required treatment with oral ciprofloxacin 750 mg twice daily and intravenous dicloxacillin for osteomyelitis complicating a fracture. After 4 weeks of treatment she complained of increasing tiredness, and was found to have a markedly raised TSH level (10 times of the upper limit of normal). Increasing the levothyroxine dose to 200 micrograms daily did not have any effect on TSH, so the dose was returned to 125 micrograms. The ciprofloxacin was then stopped, and the thyroid function tests rapidly normalised. ... 

A 36-year-old woman previously stable taking levothyroxine 100 mierograms daily and with a marked consumption of grapefruit juice (speeifie volumes not stated) had a very high TSH level even after an inerease in her levothyroxine dose to 150 micrograms daily. When she was advised to drink less grapefruit juice, her TSH fell to within the normal range. ... 

The findings from this study appear to be established. TSH levels should be closely monitored in thyroidectomy patients taking levothyroxine if they are given imatinib, anticipate the need to increase the levothyroxine dose. The authors suggest that in thyroidectomy patients the dose of levothyroxine should be doubled before starting imatinib. ... 

Fourteen patients with primary hypothyroidism had an increase in TSH levels from 1.6 to 5.4 mU/L when given ferrous sulfate 300 mg daily for 12 weeks along with their usual levothyroxine dose. The symptoms of hypothyroidism in 9 patients worsened. In another report a woman with hypothyroidism, taking levothyroxine, had a very marked rise in TSH levels when she took ferrous sulfate. Her levothyroxine dosage needed to be raised from 175 to 200 micrograms daily. ... 

In one study, patients who had been taking levothyroxine and omeprazole for a least 6 months required a modest 37% increase in the median levothyroxine dose required to suppress TSH levels to those seen before starting omeprazole. Conversely, in a pharmacokinetic study, pantoprazole did not alter the absorption of a single dose of levothyroxine. 

An interaction between levothyroxine and proton pump inhibitors is not established. The pharmacokinetic study witii pantoprazole did not reveal a change in levothyroxine absorption, whereas the study in patients who had been taking omeprazole for 6 months suggested that patients may need a modest increase in levothyroxine dose. Bear in mind the possibility of an interaction if a patient starting a proton pump inhibitor shows signs of reduced levothyroxine efficacy. Any interaction may take several months to develop. Further study is needed. 

A 75-year-old woman who had been stable taking levothyroxine 800 micrograms weekly for many years had a gradual increase in TSH levels and increasing tiredness after starting to take simvastatin 10 mg daily. After 4 months the levothyroxine dose was increased to 900 micrograms daily, but the patient s symptoms had not improved in 2 weeks and the simvastatin was stopped. The patient s symptoms gradually resolved, and the dose of levothyroxine was redueed baek to the previous level. ... 

Another patient, who had reeentiy started taking levothyroxine 50 micrograms daily, because of rising TSH levels, was also given simvastatin 10 mg daily. TSH levels continued to increase, so the simvastatin was stopped, and the TSH levels decreased to the normal range within 4 weeks without the need for an alteration in the levothyroxine dose. This patient was subsequently treated with pravastatin without a change in thyroid status. ... 

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