Synthroid (Levothyroxine Sodium):
Normal dose of Levothyroxine is usually 1.6 ug/kg. However, each individual patient must have their dose adjusted to the proper one for them. Most of the time this can be determined by measuring the serum TSH level on a blood test to make sure the TSH level is in the normal range. The TSH should not be measured more frequently than 4-6 weeks after a change in dose.
Elderly patients and those with cardiac disease should never be started on full dose therapy. In these circumstances it is best to start at a low dose (25 ug/d) and increase slowly (25 ug/d increase every 4-6 weeks).
In patients with central hypothyroidism (pituitary or hypothalamus brain problem) the serum TSH is not an accurate measure of patient's dose-response. In these circumstances your physician should be measuring a serum Free T4 level, and aim for the mid-normal range. Further adjustments of thyroid hormone dose will be dependent on clinical response (energy level, weight, temperature intolerance, etc.) specific patient conditions (cardiac disease, osteoporosis/osteopenia, etc.)