**Core Concept**
Lithium is a mood stabilizer primarily used in the treatment of bipolar disorder, including manic-depressive psychosis. It modulates neuronal signaling, particularly by inhibiting inositol monophosphate turnover and affecting neurotransmitter release, with a well-documented risk profile including hematological and endocrine side effects.

**Why the Correct Answer is Right**
Lithium is known to cause **leucocytosis** as a common laboratory finding, especially during long-term therapy. This is due to its effects on bone marrow activity and immune cell production. While it may also cause other side effects, leucocytosis is a recognized and documented hematological adverse effect, making option C accurate and clinically relevant.

**Why Each Wrong Option is Incorrect**
Option A: Lithium is **not** typically used as monotherapy for acute manic episodes; it is usually initiated during the maintenance phase after acute stabilization with other agents like benzodiazepines or antipsychotics.
Option B: Serum lithium concentration monitoring is **highly useful** for guiding dose adjustments and ensuring therapeutic levels (0.6–1.0 mmol/L), as it correlates with efficacy and toxicity.
Option D: Lithium can cause **hypothyroidism**, not hyperthyroidism; hyperthyroidism is not a known side effect and may be a red herring due to confusion with other medications.

**Clinical Pearl / High-Yield Fact**
Lithium consistently causes **leucocytosis**, which may be a marker of bone marrow stimulation — a key side effect to remember. Always monitor CBC during long-term therapy, and do not confuse this with the more common **hypothyroidism**.

✓ Correct Answer: C. Can result in leucocytosis