Does a 13 mcg Levothyroxine Dose Exist?
No, 13 mcg is not a standard commercially available levothyroxine tablet strength, and this dose would not be practical for clinical use in hypothyroidism treatment.
Standard Available Levothyroxine Tablet Strengths
Levothyroxine preparations typically provide tablets in the following strengths: 25 mcg, 50 mcg, 75 mcg, 88 mcg, 100 mcg, 112 mcg, 125 mcg, 137 mcg, 150 mcg, 175 mcg, and 200 mcg 1
The smallest commercially available tablet strength is 25 mcg, which represents the lower end of the dose range for most preparations 1
Many LT4 preparations have relatively large intervals between tablet strengths at the lower end of their dose ranges, with gaps between 25 mcg, 50 mcg, and 75 mcg tablets 1
Standard Dosing Increments for Adjustment
When adjusting levothyroxine doses, the recommended increment is 12.5-25 mcg based on the patient's current dose and clinical situation 2, 3
For elderly patients or those with cardiac disease, smaller increments of 12.5 mcg are recommended to avoid potential cardiac complications 3
For younger patients without cardiac disease, more aggressive titration using 25 mcg increments may be appropriate 3
How 12.5 mcg Adjustments Are Achieved
The 12.5 mcg dose adjustment mentioned in guidelines is typically achieved by splitting a 25 mcg tablet in half or by alternating doses on different days (e.g., alternating between 50 mcg and 75 mcg daily) 3, 4
This approach allows for fine-tuning of the dose when TSH levels indicate the need for adjustment but a full 25 mcg increase would be excessive 3
Clinical Context: Why Such Small Doses Are Rarely Needed
Initial levothyroxine dosing for young, healthy adults typically starts at approximately 1.6 mcg/kg/day, which would be 100-125 mcg for most adults 2, 3
Even for elderly patients or those with cardiac disease requiring conservative dosing, the starting dose is 25-50 mcg daily, not lower 2, 3, 5
The only clinical scenario where extremely low doses (around 1 mcg/kg) are mentioned is for central hypothyroidism in the context of immune checkpoint inhibitor toxicity, but even this would be approximately 60-80 mcg for most adults 2
Common Pitfall to Avoid
Do not attempt to compound or create custom 13 mcg doses—this is not a standard practice and would introduce unnecessary complexity and potential dosing errors 6, 1
If fine dose adjustments are needed beyond standard tablet strengths, use the alternating-day dosing strategy or tablet splitting of the 25 mcg strength 3, 1