Levothyroxine is a synthetic form of thyroxine (T4), a hormone produced by the thyroid gland. It is commonly prescribed to treat hypothyroidism, a condition where the thyroid gland doesn't produce enough thyroid hormones. Dosing of levothyroxine is critical, as both underdosing and overdosing can lead to significant health issues. The dosage is highly individualized and depends on various factors.
How Levothyroxine Dosing Works
The primary goal of levothyroxine therapy is to restore normal thyroid hormone levels in the body, alleviating symptoms of hypothyroidism and preventing long-term complications. The typical starting dose and subsequent adjustments are guided by laboratory tests (like TSH and free T4 levels) and the patient's clinical response.
Factors Influencing Dosage:
Patient Weight: This is a common starting point for dose calculation, often expressed in micrograms per kilogram (mcg/kg) of body weight per day.
Body Surface Area (BSA): In certain clinical scenarios, especially for pediatric patients or those with significant weight variations, dosage may be calculated based on body surface area (mcg/m²). BSA is a measure of the total surface area of the body.
Age: Older adults may require lower starting doses and slower titration.
Severity of Hypothyroidism: More severe cases might require higher initial doses.
Other Medical Conditions: Conditions like heart disease can necessitate cautious dosing.
Pregnancy: Thyroid hormone requirements often increase during pregnancy.
Other Medications: Certain medications can affect levothyroxine absorption or metabolism.
The Calculation Logic:
This calculator uses two common methods for initial dose estimation:
Dose per Kilogram (mcg/kg): The most straightforward method involves multiplying the patient's weight in kilograms by the prescribed dose in micrograms per kilogram.
Formula: Levothyroxine Dose (mcg) = Patient Weight (kg) × Desired Dose (mcg/kg)
Dose per Square Meter of Body Surface Area (mcg/m²): If the dose is prescribed per square meter, the patient's Body Surface Area (BSA) is used.
Formula: Levothyroxine Dose (mcg) = Body Surface Area (m²) × Desired Dose (mcg/m²)
Important Note: This calculator is a tool for estimation based on common protocols. It is NOT a substitute for professional medical advice. The final prescribed dose must be determined by a qualified healthcare provider based on a comprehensive evaluation of the patient.
Example Use Cases:
Scenario 1: A 70 kg adult patient with hypothyroidism is prescribed an initial dose of 1.6 mcg/kg/day.
Calculation: 70 kg × 1.6 mcg/kg = 112 mcg.
Scenario 2: A pediatric patient with a Body Surface Area of 1.2 m² requires a dose of 100 mcg/m²/day.
Calculation: 1.2 m² × 100 mcg/m² = 120 mcg.
function calculateLevothyroxineDose() {
var weightKgInput = document.getElementById("patientWeightKg");
var bsaInput = document.getElementById("bodySurfaceArea");
var desiredDosePerKgOrM2Input = document.getElementById("desiredDosePerKgOrM2");
var doseUnitSelect = document.getElementById("doseUnit");
var calculatedDoseElement = document.getElementById("calculatedDose");
var doseUnitsElement = document.getElementById("doseUnits");
var weightKg = parseFloat(weightKgInput.value);
var bsa = parseFloat(bsaInput.value);
var desiredDosePerKgOrM2 = parseFloat(desiredDosePerKgOrM2Input.value);
var doseUnit = doseUnitSelect.value;
calculatedDoseElement.style.color = "#28a745"; // Reset to green on new calculation
if (isNaN(weightKg) || isNaN(desiredDosePerKgOrM2)) {
calculatedDoseElement.innerText = "Error";
doseUnitsElement.innerText = "Please enter valid numbers.";
return;
}
var calculatedDose = 0;
var units = "mcg";
if (doseUnit === "mcg/kg") {
calculatedDose = weightKg * desiredDosePerKgOrM2;
units = "mcg";
if (isNaN(calculatedDose)) {
calculatedDoseElement.innerText = "Error";
doseUnitsElement.innerText = "Calculation failed.";
return;
}
} else if (doseUnit === "mcg/m²") {
if (isNaN(bsa)) {
calculatedDoseElement.innerText = "Error";
doseUnitsElement.innerText = "BSA is required for mcg/m².";
return;
}
calculatedDose = bsa * desiredDosePerKgOrM2;
units = "mcg";
if (isNaN(calculatedDose)) {
calculatedDoseElement.innerText = "Error";
doseUnitsElement.innerText = "Calculation failed.";
return;
}
}
calculatedDoseElement.innerText = calculatedDose.toFixed(2); // Display with 2 decimal places
doseUnitsElement.innerText = units;
}
// Function to toggle BSA input visibility based on selected unit
function toggleBsaInput() {
var doseUnitSelect = document.getElementById("doseUnit");
var bsaGroup = document.getElementById("bsaGroup");
if (doseUnitSelect.value === "mcg/m²") {
bsaGroup.style.display = "flex";
} else {
bsaGroup.style.display = "none";
document.getElementById("bodySurfaceArea").value = ""; // Clear BSA if not needed
}
}
// Add event listener to the select element
document.getElementById("doseUnit").addEventListener("change", toggleBsaInput);
// Initial check on page load
toggleBsaInput();