From the FDA Drug Label
The recommended starting daily dosage of levothyroxine sodium tablets in adults with primary, secondary, or tertiary hypothyroidism is based on age and comorbid cardiac conditions, as described in Table 1 For patients at risk of atrial fibrillation or patients with underlying cardiac disease, start with a lower dosage and titrate the dosage more slowly to avoid exacerbation of cardiac symptoms. Full replacement dose is 1.6 mcg/kg/day. Some patients require a lower starting dose. Titrate dosage by 12. 5 to 25 mcg increments every 4 to 6 weeks, as needed until the patient is euthyroid.
The recommended starting dose of Synthroid (levothyroxine) for patients requiring thyroid hormone replacement is 1.6 mcg/kg/day for adults with primary, secondary, or tertiary hypothyroidism. However, some patients may require a lower starting dose, especially those at risk of atrial fibrillation or with underlying cardiac disease. The dosage should be titrated by 12.5 to 25 mcg increments every 4 to 6 weeks, as needed, until the patient is euthyroid 1.
- Key considerations for dosing include:
- Patient's age
- Body weight
- Cardiovascular status
- Concomitant medical conditions
- Concomitant medications
- Co-administered food
- Specific nature of the condition being treated
- Dosage adjustments should be made based on periodic assessment of the patient's clinical response and laboratory parameters.
From the Research
The recommended starting dose of Synthroid (levothyroxine) for adult patients requiring thyroid hormone replacement is typically 1.09 μg/kg/day based on actual body weight, as suggested by the most recent study 2. This dose should be individualized based on several factors, including age, weight, and the presence of cardiovascular disease. For elderly patients or those with cardiovascular disease, a lower starting dose of 25-50 mcg daily is recommended with gradual increases every 6-8 weeks as needed, as supported by older studies 3, 4. Patients should take Synthroid on an empty stomach, 30-60 minutes before breakfast, with a full glass of water for optimal absorption. Certain medications and supplements (including calcium, iron, antacids) can interfere with absorption and should be taken at least 4 hours apart from levothyroxine, as highlighted in a study on factors contributing to high levothyroxine doses 5. Dosage adjustments are typically made based on TSH levels, which should be checked 6-8 weeks after starting therapy or changing doses. This careful dosing approach is necessary because levothyroxine has a narrow therapeutic index, and proper dosing helps avoid both under-replacement (continued hypothyroid symptoms) and over-replacement (which can cause cardiac issues and bone density loss), a consideration also relevant in the context of postoperative hypothyroidism 6. Key factors to consider in dosing include:
- Age: Older adults may require lower doses due to decreased thyroid hormone metabolism 2, 3, 4.
- Weight: Dosing should be based on actual body weight, with adjustments for ideal body weight in obese patients 2.
- Cardiovascular disease: Patients with cardiovascular disease should start with a lower dose to avoid exacerbating cardiac issues.
- Medication interactions: Certain medications can interfere with levothyroxine absorption, necessitating dose adjustments or schedule changes 5.