What is the conversion rate from levothyroxine (T4) to NP Thyroid (natural porcine thyroid hormone replacement, containing T4 and T3) in a patient with a history of thyroid hormone replacement therapy?

Conversion Rate from Levothyroxine to NP Thyroid

Direct Conversion Formula

The standard conversion is approximately 100 mcg of levothyroxine equals 60 mg (1 grain) of NP Thyroid. 1

This conversion is based on the fact that each grain (60 mg) of NP Thyroid contains 38 mcg of levothyroxine (T4) and 9 mcg of liothyronine (T3), providing a total thyroid hormone content roughly equivalent to 100 mcg of levothyroxine monotherapy 1.

Practical Conversion Algorithm

Step 1: Calculate the Initial NP Thyroid Dose

• Divide the current levothyroxine dose by 100, then multiply by 60 to get the approximate NP Thyroid dose in mg 1
• Example: A patient on 150 mcg levothyroxine would convert to approximately 90 mg NP Thyroid (150 ÷ 100 × 60 = 90 mg) 1

Step 2: Start Conservatively

• Begin with 15-30 mg NP Thyroid for patients with cardiovascular disease or long-standing hypothyroidism, regardless of their previous levothyroxine dose 1
• For otherwise healthy patients under 70 years, you can start closer to the calculated conversion dose 2
• Increase by 15 mg increments every 2-3 weeks based on clinical response and laboratory findings 1

Step 3: Monitor Appropriately

• Recheck TSH and free T4 in 6-8 weeks after conversion to assess adequacy of replacement 2, 3
• Note that T3 levels will be higher with NP Thyroid compared to levothyroxine monotherapy due to the direct T3 content 1, 4
• Target TSH within the reference range of 0.5-4.5 mIU/L with normal free T4 levels 2, 3

Critical Considerations When Converting

Why This Conversion Is Not Straightforward

The conversion from levothyroxine to NP Thyroid is more complex than a simple mathematical calculation because:

• NP Thyroid contains both T4 and T3 in a fixed 4.2:1 ratio (38 mcg T4 to 9 mcg T3 per grain), whereas levothyroxine relies entirely on peripheral conversion of T4 to T3 1
• The bioavailability of levothyroxine is approximately 80%, but absorption can be significantly decreased by proton-pump inhibitors, antacids, atrophic gastritis, and Helicobacter pylori infection 5
• Most patients on adequate levothyroxine therapy achieve normal T3 levels through peripheral conversion, making the additional T3 in NP Thyroid potentially excessive 6

Risks of Over-Replacement

Approximately 25% of patients on thyroid hormone replacement are unintentionally maintained on doses sufficient to fully suppress TSH, increasing risks for atrial fibrillation, osteoporosis, fractures, and cardiac complications 2. This risk may be higher when converting to combination T4/T3 preparations like NP Thyroid.

• Prolonged TSH suppression (<0.1 mIU/L) increases the risk of atrial fibrillation 3-5 fold, especially in patients over 60 years 2
• Bone mineral density loss and fracture risk increase, particularly in postmenopausal women 2
• Cardiovascular mortality may increase with chronic TSH suppression 2

Special Populations Requiring Modified Approach

For patients over 70 years or with cardiac disease:

• Start with 15 mg NP Thyroid daily regardless of previous levothyroxine dose 1
• Increase by 15 mg increments every 2-3 weeks with careful cardiac monitoring 1
• The appearance of angina is an absolute indication for dose reduction 1

For pregnant patients or those planning pregnancy:

• Levothyroxine monotherapy is strongly preferred over NP Thyroid during pregnancy because T3 supplementation provides inadequate fetal thyroid hormone delivery 2
• If a patient becomes pregnant while on NP Thyroid, consider switching back to levothyroxine 2

Common Pitfalls to Avoid

• Never assume a 1:1 equivalence based solely on T4 content - the additional T3 in NP Thyroid makes direct T4 content comparison misleading 1
• Do not use T3 levels to assess adequacy of replacement - T3 measurement does not add useful information in patients on thyroid hormone replacement and can be misleadingly normal even in over-replaced patients 4, 7
• Avoid adjusting doses more frequently than every 6-8 weeks before steady state is reached 2, 3
• Do not overlook the need for dose adjustment during intercurrent illness - thyroid hormone requirements can change with acute illness, medications, or physiological stress 2

Monitoring After Conversion

• TSH is the primary marker for adequacy of replacement in primary hypothyroidism, with a target range of 0.5-4.5 mIU/L 2, 3
• Free T4 can help interpret ongoing abnormal TSH levels during therapy, as TSH may take longer to normalize 2
• Most patients require 60-120 mg NP Thyroid daily for adequate replacement, with failure to respond to 180 mg suggesting non-compliance or malabsorption 1
• Once adequately treated, repeat testing every 6-12 months or with symptom changes 2, 3