Synthroid Calculator

Synthroid (Levothyroxine) Dosage Calculator

This calculator provides an estimated Synthroid (levothyroxine) dosage based on body weight and helps suggest adjustments based on current and target TSH levels. It is for informational purposes only and should not replace professional medical advice.

Understanding Synthroid (Levothyroxine) Dosage

Synthroid, also known by its generic name levothyroxine, is a synthetic thyroid hormone used to treat hypothyroidism, a condition where the thyroid gland doesn't produce enough thyroid hormone. It's also used to treat or prevent goiter (enlarged thyroid gland) and as part of the treatment for thyroid cancer.

How Levothyroxine Dosage is Determined

Levothyroxine dosage is highly individualized and depends on several factors, including:

• Body Weight: Initial dosing is often calculated based on a patient's weight, typically around 1.6 mcg per kilogram of body weight per day for adults with primary hypothyroidism.
• TSH Levels: Thyroid Stimulating Hormone (TSH) is the primary lab test used to monitor thyroid function and guide dosage adjustments. The goal is usually to bring TSH levels into a normal or target range, which can vary depending on the individual's age, underlying condition, and other health factors.
• Patient's Age and Health Status: Older adults or those with heart conditions may start on lower doses to avoid adverse effects.
• Cause of Hypothyroidism: Dosage requirements can differ for primary hypothyroidism, post-thyroidectomy, or central hypothyroidism.
• Other Medications: Certain medications can interfere with levothyroxine absorption or metabolism, requiring dose adjustments.
• Symptoms: While TSH is key, a doctor will also consider a patient's symptoms of hypothyroidism (e.g., fatigue, weight gain, cold intolerance) when adjusting dosage.

The Role of TSH in Dosage Adjustment

TSH is produced by the pituitary gland and signals the thyroid to produce more thyroid hormone. When thyroid hormone levels are low (hypothyroidism), the pituitary releases more TSH to stimulate the thyroid. Conversely, when thyroid hormone levels are high (hyperthyroidism or over-replacement), TSH levels drop. Therefore, TSH levels are an excellent indicator of whether the current levothyroxine dose is adequate.

Doctors typically aim for a TSH level within a specific target range, often between 0.5 and 2.5 mIU/L for most adults with primary hypothyroidism. If TSH is too high, the dose may need to be increased. If TSH is too low, the dose may need to be decreased.

Using the Synthroid Dosage Calculator

This calculator provides an estimate based on common clinical guidelines:

• Estimated Initial Dose: Calculated using a standard factor of 1.6 mcg/kg/day for primary hypothyroidism.
• Suggested Dose Adjustment: This is estimated based on the difference between your current TSH and your target TSH. A common approximation is that a change of 12.5 mcg/day of levothyroxine can alter TSH by approximately 1 mIU/L. This calculator uses this principle to suggest an adjustment.
• New Estimated Daily Dose: This combines your current dose with the suggested adjustment.

Important Considerations:

• Levothyroxine comes in various strengths (e.g., 25, 50, 75, 88, 100, 112, 125, 137, 150, 175, 200, 300 mcg). Your doctor will prescribe the closest available strength or a combination of strengths to achieve the desired daily dose.
• It takes about 4-6 weeks for levothyroxine to reach steady-state levels in the body, so TSH levels are typically re-checked after this period following a dose change.
• Always take levothyroxine on an empty stomach, usually 30-60 minutes before breakfast, and separate it from other medications or supplements (especially calcium, iron, and antacids) by several hours.

Example Calculation:

Let's say a patient weighs 70 kg, has a Current TSH of 5.5 mIU/L, a Target TSH of 1.5 mIU/L, and is currently taking 75 mcg/day of Synthroid.

• Estimated Initial Dose: 70 kg * 1.6 mcg/kg = 112 mcg/day
• TSH Difference: 5.5 mIU/L (Current) – 1.5 mIU/L (Target) = 4.0 mIU/L
• Suggested Dose Adjustment: 4.0 mIU/L * 12.5 mcg/mIU/L = +50 mcg/day
• New Estimated Daily Dose: 75 mcg/day (Current) + 50 mcg/day (Adjustment) = 125 mcg/day

In this example, the calculator would suggest increasing the dose from 75 mcg to 125 mcg per day.

Disclaimer: This Synthroid Dosage Calculator is for educational and informational purposes only. It is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition or treatment and before undertaking a new health care regimen. Do not disregard professional medical advice or delay in seeking it because of something you have read on this website. Dosage adjustments should only be made under the guidance of a healthcare professional.

Estimated Initial Dose:

Suggested Dose Adjustment:

New Estimated Daily Dose:

Note: Dosage adjustments should always be rounded to available pill strengths (e.g., 12.5, 25, 50 mcg) and made under the guidance of a healthcare professional.