What is the equivalent dose of levothyroxine (T4) for a 235lb female patient taking NP Thyroid (thyroid desiccated) 120mg once daily?

Converting NP Thyroid 120 mg to Levothyroxine for a 235lb Female

For a 235lb female patient converting from NP Thyroid 120 mg to levothyroxine monotherapy, the appropriate starting dose would be approximately 150 mcg of levothyroxine daily.

Conversion Rationale

• NP Thyroid is a desiccated thyroid extract containing both T4 and T3 in an approximate ratio of 4:1 1
• A 120 mg dose of desiccated thyroid extract typically contains approximately 76 mcg of T4 and 18 mcg of T3 1
• When converting to levothyroxine monotherapy, the T3 component must be accounted for in the total T4 equivalent dose 1

Dose Calculation Considerations

• For most patients, full replacement levothyroxine dosing is approximately 1.6 mcg/kg of actual body weight per day 2, 3
• For a 235lb patient (approximately 107 kg), this would calculate to approximately 171 mcg daily 3
• However, for obese patients, the dose requirement per kg is typically lower than for normal-weight individuals 4
• For patients with BMI ≥30 kg/m², the appropriate levothyroxine dose is approximately 1.39 mcg/kg of actual body weight 4
• Using this adjusted calculation for obesity, the dose would be approximately 149 mcg (107 kg × 1.39 mcg/kg) 4

Monitoring and Adjustment Protocol

• After initiating levothyroxine therapy, check TSH and free T4 in 6-8 weeks to assess adequacy of replacement 5
• Adjust the dose in 12.5-25 mcg increments based on TSH results 5
• Target a TSH within the reference range, with consideration for the lower half of the reference range if symptoms persist despite normal TSH 6, 5
• Once stabilized, monitor thyroid function annually if the patient remains clinically stable 5

Special Considerations

• Patients previously on combination therapy with T3 may experience transient hypothyroid symptoms when switching to T4-only therapy, even with appropriate dosing 1
• Morning administration on an empty stomach (30-60 minutes before breakfast) optimizes absorption 2
• Certain medications and supplements (calcium, iron, antacids) can interfere with levothyroxine absorption and should be separated by at least 4 hours 6
• If the patient remains symptomatic despite normalized TSH on levothyroxine monotherapy, reconsideration of combination therapy might be warranted after an adequate trial period 1

Potential Pitfalls and Caveats

• Avoid excessive dosing that could lead to subclinical hyperthyroidism (TSH <0.1 mIU/L), which increases risk of atrial fibrillation and bone loss, particularly in elderly patients 6, 2
• Bioequivalence can differ among generic and brand name levothyroxine products; consistency in using the same product is recommended 2
• Weight changes may necessitate dose adjustments; monitor more frequently during periods of significant weight change 4
• If the patient has thyroid cancer history, target TSH may need to be lower, but this would require specialist input 5