Repeat the lithium level in one week to confirm accuracy before making any clinical decisions or medication changes.

A lithium level of 1.8 mEq/L with neurological symptoms is toxicity, not a borderline result to recheck. Delaying assessment by a week is dangerous.

Increase oral fluid intake to promote renal lithium excretion while continuing the current dose and monitoring symptoms closely.

Continuing the current dose with a toxic level and active symptoms is contraindicated. Lithium should be held immediately.

Assess renal function, hydration status, and recent medication changes, particularly drugs that reduce lithium clearance.

Lithium toxicity (levels >1.5 mEq/L with symptoms) requires immediate assessment of the precipitating cause. The most common causes in stable patients are dehydration, renal function changes, and drug interactions that reduce lithium clearance — particularly NSAIDs, ACE inhibitors, ARBs, and thiazide diuretics. In a 68-year-old, age-related renal decline may also contribute. Hold lithium, assess renal function and hydration, review the medication list for interactions, and determine whether the patient requires emergency medical management.

Obtain a TSH level because the tremor and cognitive changes likely reflect lithium-induced hypothyroidism rather than acute toxicity.

While lithium causes hypothyroidism, the acute presentation with confusion and a level of 1.8 mEq/L points to toxicity, not a chronic thyroid effect.