What symptom is a 17-year-old boy with bipolar II disorder, therapeutic lithium (lithium) levels, and normal renal function studies, most likely to experience?

Lithium-Induced Polyuria and Urinary Frequency in Adolescent with Bipolar II Disorder

Urinary frequency is the most likely symptom present in this 17-year-old boy with bipolar II disorder on lithium therapy, despite normal renal function studies. This is due to lithium's well-documented effect on renal tubular function, which can occur early in treatment even with therapeutic levels and normal laboratory values.

Mechanism of Lithium-Induced Urinary Frequency

Lithium commonly affects renal tubular function by:

• Impairing urinary concentrating ability in the kidneys
• Causing resistance to antidiuretic hormone (ADH) action
• Leading to nephrogenic diabetes insipidus in some patients 1

This effect can occur relatively early in treatment, even when other laboratory values remain normal. The patient's normal renal function studies do not rule out this common side effect, as standard renal function tests primarily assess glomerular function rather than tubular function.

Clinical Presentation

Patients with lithium-induced renal tubular dysfunction typically present with:

• Urinary frequency (most common early manifestation)
• Polyuria (urine output >3 L/day)
• Polydipsia (increased thirst)
• Nocturia (nighttime urination)

These symptoms reflect the kidney's decreased ability to concentrate urine, resulting in increased urine volume and frequency of urination.

Assessment of Renal Effects in Lithium Treatment

The American Academy of Child and Adolescent Psychiatry recommends:

• Baseline laboratory assessment before lithium initiation, including complete blood count, thyroid function tests, urinalysis, blood urea nitrogen, creatinine, and serum calcium 1
• Regular monitoring of renal function every 3-6 months once a stable lithium dose is achieved 1

Why Other Symptoms Are Less Likely

1. Erectile dysfunction: While sexual dysfunction can occur with some psychotropic medications, it is not a common early side effect of lithium in adolescents.

2. Nonrestorative sleep: Although sleep disturbances can occur with bipolar disorder and some medications, this is not a characteristic side effect of lithium.

3. Painless hematuria: This would suggest glomerular damage, which is a rare and late complication of lithium therapy, not expected in a young patient with normal renal function studies.

4. Scleral icterus: This would indicate liver dysfunction or hemolysis, which is not associated with lithium therapy. The patient's liver function studies are normal.

Monitoring and Management

For patients on lithium with urinary frequency:

• Monitor fluid intake and output
• Assess for dehydration risk, which could lead to lithium toxicity 2
• Consider measuring urine osmolality and specific gravity to evaluate concentrating ability 1
• Ensure adequate hydration while avoiding excessive fluid intake

Long-Term Considerations

While the patient currently has normal renal function tests, long-term lithium therapy requires vigilance:

• Progressive changes in renal function, even within normal range, indicate need for treatment reevaluation 2
• About one-third of patients on lithium for 10-29 years develop evidence of chronic renal failure 3
• Regular monitoring of kidney function by measuring serum creatinine and glomerular filtration rate is necessary throughout treatment 4

In summary, urinary frequency due to lithium's effect on renal tubular function is the most likely symptom in this adolescent patient, despite normal renal function studies. This represents an early manifestation of lithium's known effects on the kidney that can occur even with therapeutic drug levels.