Which of the following is a common side effect of lithium?

Lithium, a medication widely used for the treatment of bipolar disorder, is known to influence various physiological processes in the body. One of the significant side effects associated with long-term use of lithium is thyroid dysfunction. This occurs because lithium can inhibit the synthesis and release of thyroid hormones, leading to conditions such as hypothyroidism. It is important for patients on lithium to have their thyroid function monitored regularly, as this side effect can affect metabolism and overall health.

In contrast, while dry mouth, gastrointestinal upset, and hyperkalemia can occur with many medications, they are less specifically associated with lithium. Dry mouth is a common side effect of various drugs but not directly linked to lithium. Gastrointestinal upset may also happen but is not a prominent or distinctive side effect of lithium therapy compared to thyroid dysfunction. Hyperkalemia, or elevated potassium levels, is not a primary concern with lithium usage; instead, lithium is more commonly associated with renal function effects. Thus, the link between lithium and thyroid dysfunction is a noteworthy aspect of its pharmacological profile.