Can I start lithium 400 mg in the afternoon without first performing baseline investigations such as renal function tests, thyroid function tests, electrolyte panel, complete blood count, pregnancy test, and ECG if indicated?

No, You Cannot Start Lithium Without Baseline Investigations

You must obtain baseline laboratory investigations before initiating lithium therapy, regardless of the dose or time of day. Starting lithium 400 mg in the afternoon without baseline testing violates established safety standards and exposes patients to preventable harm.

Required Baseline Investigations Before Starting Lithium

The following tests are mandatory before the first lithium dose 1:

• Complete blood count (CBC) - to document baseline hematologic parameters 1
• Renal function tests - blood urea nitrogen, serum creatinine with estimated glomerular filtration rate (eGFR) 1
• Thyroid function tests - thyroid-stimulating hormone (TSH) at minimum 1
• Serum electrolytes - including calcium and magnesium 1
• Fasting blood glucose or glycated hemoglobin - to assess diabetes risk 1
• Urinalysis - to establish baseline renal parameters 1
• Pregnancy test - for women of childbearing potential 2, 1

Why Baseline Testing Is Non-Negotiable

Renal Function Assessment

Lithium is entirely renally excreted and has significant nephrotoxic potential even at therapeutic levels 3. Patients with pre-existing renal impairment (eGFR <60 mL/min/1.73 m²) require dose adjustments and more frequent monitoring 1. Starting lithium without knowing baseline renal function can lead to rapid toxicity accumulation, particularly since lithium clearance is considerably reduced in renal impairment 1.

Thyroid Function Screening

Women under 60 years are at particularly high risk for lithium-induced thyroid dysfunction, with hypothyroidism developing in approximately 20-30% of patients on long-term lithium 1. Baseline TSH is essential to distinguish pre-existing thyroid disease from treatment-emergent dysfunction 3, 4. Thyroid autoimmunity is found in excess among patients with affective disorders even before lithium exposure 4.

Calcium and Parathyroid Function

Hypercalcemia occurs in approximately 25% of long-term lithium patients 1. Baseline calcium measurement allows detection of treatment-emergent hypercalcemia, which may require parathyroid hormone assessment 1, 3.

Pregnancy Screening

Lithium is a known teratogen 2. A pregnancy test must be performed before initiating therapy in women of childbearing potential to allow informed risk-benefit discussion 2, 1.

Common Pitfalls to Avoid

Failure to obtain baseline values makes it impossible to determine whether abnormalities are treatment-emergent or pre-existing 1. This creates medicolegal risk and prevents appropriate clinical decision-making when laboratory abnormalities emerge during treatment.

Do not assume a "low" dose of 400 mg is safe without monitoring. While 400 mg may produce subtherapeutic levels in some patients, individual variation in renal clearance means this dose could produce toxic levels in patients with undetected renal impairment 1.

The timing of administration (afternoon vs. morning) is irrelevant to the requirement for baseline testing. All patients require baseline investigations regardless of dosing schedule 1.

Clinical Algorithm for Lithium Initiation

1. Order and review all baseline investigations listed above 1
2. Confirm eGFR ≥60 mL/min/1.73 m² for standard dosing; if <60, consider alternative agents or plan for intensified monitoring 1
3. Verify TSH is within normal range; if abnormal, address thyroid dysfunction before lithium initiation 1, 4
4. Confirm negative pregnancy test in women of childbearing potential 2, 1
5. Review current medications for interactions (NSAIDs, ACE inhibitors, ARBs, thiazide diuretics) that increase lithium toxicity risk 1
6. Only after all baseline results are reviewed and acceptable, initiate lithium therapy 1

Post-Initiation Monitoring Requirements

After starting lithium, check serum lithium levels twice weekly until both the level and clinical condition stabilize during the acute treatment phase 1. Recheck lithium level at 7-8 days when steady state is achieved 1. Continue monitoring lithium levels every 3 months and renal function, electrolytes, thyroid function, and calcium every 6 months during maintenance therapy 1.