Nervousness, excitability, tremor, muscle weakness, fatigue, cramps; sweating, flushing, heat intolerance, headache, fever, insomnia, tachycardia, palpitations, restlessness, anginal pain, HTN, severe depression, difficulty in sleeping, excessive wt loss; menstrual irregularities; diarrhoea, vomiting, psychosis or agitation. Increased bone resorption and reduced bone mineral density, especially in post-menopausal women; elevated LFT.
Potentially Fatal: Thyrotoxic crisis including convulsions, cardiac arrhythmia, heart failure, coma.
Pregnancy Category
Name :
Not Classified
Description
FDA has not yet classified the drug into a specified pregnancy category.
Mode of Action
Levothyroxine Na is a synthetic form of thyroxine which increases the basal metabolic rate (BMR) and the utilisation and mobilisation of glycogen stores and stimulates protein synthesis. It is also involved in normal metabolism, growth and development. These effects are mediated at the cellular level by the thyroxine metabolite, tri-iodothyronine.
Interaction
Reduced absorption w/ iron, antacids, bile acid sequestrants, colestyramine, simeticone, Ca carbonate, sucralfate, cation exchange resins. Reduced tri-iodothyronine serum levels w/ amiodarone and propranolol. Reduced serum levels of thyroxine w/ carbamazepine, phenytoin, phenobarbital, rifampicin, lithium, oestrogens, sertraline. Androgens may decrease levothyroxine-binding globulins serum levels. May alter requirements of antidiabetic drugs. Increased risk of significant HTN and tachycardia w/ ketamine. Increased metabolic demands w/ sympathomimetics (e.g. epinephrine). May increase anticoagulant effect of warfarin.
Pregnancy Category Note
Pregnancy
Experience with levothyroxine use in pregnant women, including data from post-marketing studies, have not reported increased rates of major birth defects or miscarriages; there are risks to mother and fetus associated with untreated hypothyroidism in pregnancy; since TSH levels may increase during pregnancy, TSH should be monitored and levothyroxine dosage adjusted during pregnancy; untreated maternal hypothyroidism may have adverse effect on fetal neurocognitive development
Maternal hypothyroidism during pregnancy is associated with a higher rate of complications, including spontaneous abortion, gestational hypertension, pre-eclampsia, stillbirth, and premature delivery; untreated maternal hypothyroidism may have adverse effect on fetal neurocognitive development
Pregnancy may increase thyroid hormone requirements. serum TSH level should be monitored and the dosage adjusted during pregnancy; since postpartum TSH levels are similar to preconception values, the dosage should return to the pre-pregnancy dose immediately after delivery
Lactation
Limited published studies report that levothyroxine is present in human milk; however, there is insufficient information to determine effects of levothyroxine on breastfed infant and no available information on effects of levothyroxine on milk production; adequate levothyroxine treatment during lactation may normalize milk production in hypothyroid lactating mothers; developmental and health benefits of breastfeeding should be considered along with mother?s clinical need for levothyroxine and any potential adverse effects on breastfed infant from levothyroxine or from underlying maternal condition
Adult Dose
Oral
Mild Hypothyroidism
1.7 mcg/kg or 100-125 mcg PO qDay; not to exceed 300 mcg/day
>50 years (or <50 yr with CV disease)
Usual initial dose: 25-50 mcg/day
May adjust dose by 12.5-25 mcg q6-8Week
>50 years with CV disease
Usual initial dose: 12.5-25 mcg PO qDay
May adjust dose by 12.5-25 mcg q4-6weeks until patient becomes euthyroid and serum TSH concentration normalized; adjustments q6-8weeks also used
Dose range: 100-125 mcg PO qDay
Severe Hypothyroidism
Initial: 12.5-25 mcg PO qDay
Adjust dose by 25 mcg/day q2-4Week PRN
Subclinical Hypothyroidism
Initial: 1 mcg/kg PO qDay may be adequate, OR
If replacement therapy not initiated, monitor patient annually for clinical status
TSH suppression
For thyrotropin-dependent well-differentiated thyroid cancer: Doses >2 mcg/kg/day may be given as a single dose to suppress TSH to <0.1 MIU/L.
For benign nodules and nontoxic multinodular goitre: Target TSH is generally higher at 0.1-0.5 MIU/L for nodules and 0.5-1 MIU/L for multinodular goitre.
Child Dose
Oral
Hypothyroidism
Age 1-3 months
10-15 mcg/kg/day PO
Use lower starting dose (25 mcg/day) if patient at risk of cardiac failure; if initial serum T4 lower than 5 mcg/dL begin treatment at higher dose (50 mcg/day)
Age 3-6 months
8-10 mcg/kg/day PO, OR
25-50 mcg/day PO
Age 6-12 months
6-8 mcg/kg/day PO, OR
50-75 mcg/day PO
Age 1-5 years
5-6 mcg/kg/day PO, OR
75-100 mcg/day PO
Age 6-12 years
4-5 mcg/kg/day PO, OR
100-125 mcg/day PO
>12 years
2-3 mcg/kg/day PO, OR
150 mcg/day PO
Start children with severe or chronic hypothyroidism at 25 mcg/day; adjust dose by 25 mcg qweek
Renal Dose
N/A
Administration
Should be taken on an empty stomach with full glass of water. Take on an empty stomach ?-1 hr before meals.
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