Desiccated Thyroid Dosing for Hypothyroidism
Desiccated thyroid extract is not the preferred initial treatment for hypothyroidism; levothyroxine monotherapy should be used instead, with typical starting doses of 1.6 mcg/kg/day for most patients or 25-50 mcg/day for elderly patients and those with cardiac disease. 1, 2
Why Levothyroxine is Preferred Over Desiccated Thyroid
Levothyroxine (synthetic T4) is the preparation of choice for most patients with hypothyroidism because it provides uniform levels of both T4 and T3 without diurnal variation through physiologic conversion 3, 4
Desiccated thyroid extract contains both T4 and T3 in a fixed ratio of approximately 4:1, which frequently causes serum T3 to rise to supranormal values during the absorption phase, associated with palpitations 4, 5
Desiccated thyroid extract remains outside formal FDA oversight, and consistency of T4 and T3 contents is monitored by manufacturers only, raising quality control concerns 5
When Desiccated Thyroid May Be Considered
DTE can be considered only as a trial for patients who remain symptomatic despite adequate levothyroxine therapy and normalized TSH levels 5
The mean daily dose of DTE needed to normalize serum TSH contains approximately 11 mcg T3, though some patients may require higher doses 5
Newly diagnosed hypothyroid patients should always be treated with levothyroxine first, not desiccated thyroid 5
Standard Levothyroxine Dosing Algorithm
For Patients <70 Years Without Cardiac Disease:
- Start with full replacement dose of approximately 1.6 mcg/kg/day 1, 3, 2
- This approach reduces the need for follow-up visits and repeated laboratory testing for dose titration 3
For Patients >70 Years or With Cardiac Disease:
- Start with a lower dose of 25-50 mcg/day and titrate gradually 1, 2, 6
- Even minor over-replacement should be avoided because of the risk of cardiac events 3
- Levothyroxine can and should be continued in patients receiving treatment for coronary artery disease, but careful dose titration is essential 3
Monitoring and Dose Adjustment
- Monitor TSH every 6-8 weeks while titrating hormone replacement 1, 6
- Target TSH should be within the reference range (0.5-4.5 mIU/L), with a normal TSH level of 1-2 mU/L considered optimal 1, 3
- Once adequately treated with a stable dose, repeat TSH testing every 6-12 months 1
Critical Pitfalls to Avoid
- Approximately 25% of patients on levothyroxine are unintentionally maintained on doses sufficient to fully suppress TSH, increasing risks for osteoporosis, fractures, and cardiac complications 1
- Chronic over-replacement may induce osteoporosis, particularly in postmenopausal women, and should be avoided 3
- TSH suppression (<0.1 mIU/L) carries risk of atrial fibrillation and bone loss, especially in elderly patients 4, 1
Alternative Combination Therapy (If Levothyroxine Alone Fails)
- For patients who remain symptomatic on LT4 therapy despite normalized TSH, reduce the LT4 dose by 25 mcg/day and add 2.5-7.5 mcg liothyronine (LT3) once or twice daily 5
- This synthetic combination therapy is preferred over desiccated thyroid extract due to better quality control and dosing precision 5
- Trials following almost 1000 patients for almost 1 year indicate that therapy with LT4+LT3 can restore euthyroidism while maintaining normal serum TSH 5