If a patient takes lithium at bedtime (HS), when should the lithium level and accompanying renal and thyroid labs be drawn?

Lithium Lab Draw Timing for Bedtime Dosing

For patients taking lithium at bedtime (HS), blood samples for lithium levels should be drawn 12 hours after the last dose, which means drawing labs in the morning before the next scheduled evening dose.

Timing for Lithium Levels

The FDA-approved lithium labeling explicitly states: "Blood samples for serum lithium determination should be drawn immediately prior to the next dose when lithium concentrations are relatively stable (i.e., 8-12 hours after the previous dose)" 1. This 12-hour standard applies regardless of whether lithium is given once daily at bedtime or in divided doses throughout the day.

Key Considerations:

• For bedtime dosing: If a patient takes lithium at 10 PM, draw the lithium level at 10 AM the next morning (before any morning dose if applicable)
• The 12-hour standard is critical because virtually all published lithium dosing data and therapeutic ranges (0.6-1.2 mEq/L for maintenance) are based on 12-hour trough levels 1
• Research confirms that at 12 hours post-dose, blood levels for immediate-release and extended-release formulations are essentially identical, making this the ideal comparison point 2

Important Caveats About Extended-Release Formulations

If the patient is taking once-daily extended-release lithium at bedtime, there is debate about optimal timing:

• Some evidence suggests 24-hour levels (drawing just before the next bedtime dose) may be more appropriate for once-daily extended-release formulations 3
• However, the 12-hour standard remains most comparable to established therapeutic data 2
• Practical recommendation: For once-daily extended-release lithium at bedtime, draw at 12 hours (morning) but be aware the level may be approximately 1.3 times higher than a true 24-hour trough 3

Accompanying Labs Timing

Renal and thyroid monitoring labs should be drawn at the same time as the lithium level (12 hours post-dose, in the morning for bedtime dosing):

• Renal function: Serum creatinine, BUN, electrolytes (sodium, potassium)
• Thyroid function: TSH, free T4, and thyroid peroxidase antibodies 4
• Additional monitoring: Calcium levels (lithium can cause hyperparathyroidism) 5

Monitoring Frequency:

• Acute phase: Twice weekly until stable 1
• Maintenance: Every 2 months minimum for uncomplicated cases 1
• Annual screening: TSH, free T4, glucose, HbA1c, complete blood count, B12 levels 4

Common Pitfalls to Avoid

1. Non-compliance with 12-hour timing: Recent data shows approximately 50% of lithium levels are drawn outside the recommended 10-14 hour window 6. Blood draws >14 hours after dosing yield falsely low levels (0.41 vs 0.64 mEq/L) 6

2. Forgetting to verify timing: Always document and verify when the patient actually took their last lithium dose—don't assume compliance

3. Starting thyroid replacement before steroids: In patients with both adrenal insufficiency and hypothyroidism (which can co-occur with lithium), always start corticosteroids before thyroid hormone to avoid precipitating adrenal crisis 7

4. Ignoring drug interactions: NSAIDs, ACE inhibitors, and diuretics can significantly increase lithium levels—monitor more frequently when these are started or stopped 1