What laboratory test should be routinely monitored in patients taking lithium?

In patients taking lithium, routine monitoring of thyroid stimulating hormone (TSH) levels is crucial due to lithium's potential effects on thyroid function. Lithium can induce hypothyroidism in some individuals by inhibiting the synthesis and release of thyroid hormones. Consequently, monitoring TSH helps in early identification and management of any thyroid dysfunction that may develop during treatment.

While other choices involve important laboratory tests, they serve different monitoring purposes. Electrolytes are monitored to assess kidney function and fluid balance, particularly since lithium can affect renal performance. Serum creatinine is specifically monitored to evaluate renal function, as lithium is nephrotoxic and can lead to renal impairment. A complete metabolic panel includes a range of tests that provide a broader view of overall health and metabolic function but does not focus specifically on the side effects associated with lithium therapy. Thus, TSH monitoring is the most relevant choice when specifically considering the effects of lithium on thyroid health.