Calculate the appropriate Synthroid (levothyroxine sodium) dosage based on patient weight and target TSH levels. This tool is for informational purposes and should not replace professional medical advice.
Initial/Standard (1.6 mcg/kg/day)
Elderly/Cardiac Issues (1.0 mcg/kg/day)
Hypothyroid Infants (1.7 mcg/kg/day)
Children (0.9 mcg/kg/day)
Pregnant Women (1.2 mcg/kg/day)
Custom
Your estimated Synthroid dose will appear here.
Understanding Synthroid Dosing
Synthroid, the brand name for levothyroxine sodium, is a synthetic form of the thyroid hormone thyroxine (T4). It is primarily used to treat hypothyroidism, a condition where the thyroid gland does not produce enough thyroid hormones. The goal of Synthroid therapy is to restore normal metabolic function by replacing or supplementing the body's natural thyroid hormone.
Factors Influencing Synthroid Dosage
Determining the correct Synthroid dosage is a complex process that depends on several individual patient factors:
Body Weight: Dosage is often calculated based on a patient's weight in kilograms (kg). This provides a baseline for hormone replacement.
Age: Older adults or those with cardiovascular conditions may require lower starting doses to prevent cardiac strain. Infants and children often need higher doses relative to their weight due to growth and metabolism.
Severity of Hypothyroidism: The degree to which thyroid hormone levels are deficient influences the initial dose required.
Other Medical Conditions: Conditions like heart disease, diabetes, or adrenal insufficiency can affect how the body processes levothyroxine and may necessitate dosage adjustments.
Pregnancy: Thyroid hormone requirements often increase during pregnancy, requiring careful monitoring and dosage adjustments.
Other Medications: Certain medications can interfere with levothyroxine absorption or metabolism.
Target TSH Levels: While the standard TSH (Thyroid-Stimulating Hormone) target is often between 0.4-4.0 mIU/L, individualized targets may be set by a physician, especially in cases of differentiated thyroid cancer or other specific conditions.
The Calculation Behind the Dosing
This calculator uses a common formula for estimating Synthroid dosage:
The Adjustment Factor is crucial and represents the typical dose of levothyroxine required per kilogram of body weight per day. Different patient populations have different recommended adjustment factors:
Standard/Initial Adult Dosing: Commonly around 1.6 mcg/kg/day.
Elderly or Patients with Cardiac Issues: Typically start with a lower dose, around 1.0 mcg/kg/day, to avoid cardiac side effects.
Hypothyroid Infants: Require higher doses, often around 1.7 mcg/kg/day, due to higher metabolic demands.
Children: Dosing is typically around 0.9 mcg/kg/day, but can vary significantly with age and growth spurts.
Pregnant Women: Requirements often increase, with adjustments potentially reaching 1.2 mcg/kg/day or more, requiring close monitoring.
The Target TSH Level is not directly used in this initial *calculation* of the starting dose, but it is the primary metric used by physicians to *adjust* the dose over time. Once a patient starts Synthroid, blood tests are performed to measure TSH levels, and the dosage is fine-tuned until the TSH level falls within the desired therapeutic range.
How to Use This Calculator
Enter the patient's weight in kilograms.
Select the appropriate adjustment factor based on the patient's profile (age, health conditions, pregnancy status). If you have a specific, doctor-recommended custom value, select "Custom" and enter it.
Click "Calculate Dose".
Disclaimer: This calculator provides an *estimated* starting dose. It is imperative that any Synthroid dosage be prescribed and monitored by a qualified healthcare professional. Self-medication or dosage adjustments based solely on this calculator can be dangerous.
function calculateSynthroidDose() {
var weightInput = document.getElementById("patientWeightKg");
var adjustmentFactorSelect = document.getElementById("adjustmentFactor");
var customAdjustmentFactorInput = document.getElementById("customAdjustmentFactor");
var resultDiv = document.getElementById("result");
var weightKg = parseFloat(weightInput.value);
var adjustmentFactorValue;
if (adjustmentFactorSelect.value === "custom") {
adjustmentFactorValue = parseFloat(customAdjustmentFactorInput.value);
} else {
adjustmentFactorValue = parseFloat(adjustmentFactorSelect.value);
}
// Clear previous results and error messages
resultDiv.innerHTML = "";
// Input validation
if (isNaN(weightKg) || weightKg <= 0) {
resultDiv.innerHTML = "Please enter a valid patient weight (in kg).";
resultDiv.style.color = "#dc3545"; // Red for errors
return;
}
if (isNaN(adjustmentFactorValue) || adjustmentFactorValue <= 0) {
resultDiv.innerHTML = "Please select or enter a valid adjustment factor.";
resultDiv.style.color = "#dc3545"; // Red for errors
return;
}
// Calculation
var estimatedDose = weightKg * adjustmentFactorValue;
// Display result
resultDiv.innerHTML = "Estimated Daily Synthroid Dose: " + estimatedDose.toFixed(2) + " mcg";
resultDiv.style.color = "#28a745"; // Green for success
}
// Show/hide custom input based on selection
document.getElementById("adjustmentFactor").onchange = function() {
var customInput = document.getElementById("customAdjustmentFactor");
if (this.value === "custom") {
customInput.style.display = "block";
customInput.placeholder = "Enter custom value";
} else {
customInput.style.display = "none";
}
};