Converting from Synthroid 100 mcg to NP Thyroid
I do not recommend switching from Synthroid (levothyroxine) 100 mcg to NP Thyroid (desiccated thyroid). The available guidelines explicitly state that levothyroxine monotherapy is the standard of care for hypothyroidism, and desiccated thyroid preparations have not been systematically studied 1.
Why This Switch Is Not Recommended
The 2012 AACE/ATA clinical practice guidelines note that desiccated thyroid has not been systematically studied and that the content of thyroid hormone and the T4:T3 ratio may vary depending on the brand and whether it is of porcine or bovine origin 1. This variability creates unpredictable dosing and potential for over- or under-treatment, which can lead to:
- Cardiovascular complications (atrial fibrillation, increased cardiac mortality in elderly) from overtreatment 1
- Accelerated bone loss in postmenopausal women from excess thyroid hormone 1
- Suboptimal TSH control affecting morbidity and quality of life
If You Must Convert (Against Guideline Recommendations)
Should you proceed despite the lack of evidence supporting this switch, understand that:
- NP Thyroid contains approximately a 4:1 ratio by weight of T4:T3 1
- Traditional conversion ratios suggest approximately 60-65 mg (1 grain) of desiccated thyroid ≈ 100 mcg levothyroxine, but this is based on historical data, not rigorous clinical trials
- The higher T3 content in desiccated thyroid means patients often experience supraphysiologic T3 peaks, which may cause symptoms of hyperthyroidism
Critical Monitoring Requirements
If converting:
- Start with 60 mg (1 grain) NP Thyroid daily as an approximate equivalent
- Check TSH and free T4 in 4-8 weeks 1
- Adjust in small increments (15-30 mg) based on clinical response
- Monitor closely for signs of overtreatment: palpitations, tremor, anxiety, weight loss
Important Caveats
Elderly patients and those with cardiac disease are at highest risk for adverse events from this switch 1, 2. The NP Thyroid label specifically warns to use great caution in patients with suspected cardiovascular compromise, starting with only 15-30 mg in these populations 2.
The goal is achieving euthyroid state with normalized TSH 1, 3. Levothyroxine monotherapy achieves this predictably in the vast majority of patients when dosed appropriately (average 1.7 mcg/kg/day for full replacement, or approximately 100-125 mcg for a 70 kg adult) 1, 3.
Bottom Line
Remain on Synthroid 100 mcg daily unless there is a compelling, documented reason for switching that outweighs the risks of using a non-standardized preparation. If the patient is symptomatic despite normal TSH on levothyroxine, address compliance, timing of administration, drug interactions, and consider adjusting the levothyroxine dose to target TSH in the lower half of the reference range 1 rather than switching to an unstudied alternative.