Lithium requires more routine monitoring than most psychiatric medications. The standard target blood level for maintenance therapy is 0.60 to 0.80 mmol/L, and because the gap between a therapeutic dose and a toxic one is narrow, regular testing of blood levels, kidney function, thyroid hormones, calcium, and heart rhythm forms the backbone of safe, long-term use.
Baseline Tests Before Starting
Before your first dose, your provider will order a set of labs to check for pre-existing conditions that could complicate treatment. These typically include a basic kidney panel (BUN and creatinine), a thyroid panel (TSH, with free T4 if needed), serum calcium, and a urinalysis. A pregnancy test is standard for anyone who could become pregnant. If you have risk factors for heart disease, such as diabetes, high blood pressure, high cholesterol, or a smoking history, you’ll also get a baseline ECG.
This initial snapshot matters because lithium can affect several of the same organs over time. Without knowing your starting point, it becomes much harder to tell whether a later lab result reflects a lithium side effect or something that was already there.
Serum Lithium Levels
Blood draws for lithium levels are done 12 hours after your last dose, usually first thing in the morning. During the first year of treatment, expect testing roughly every three months. After a year of stable levels in the 0.40 to 0.79 mmol/L range, most guidelines allow extending to every six months. If your level sits in the higher range of 0.80 to 0.99 mmol/L, the three-month interval should continue.
For adults over 65, some guidelines recommend staying at every three months indefinitely, though recent analysis suggests that people with a solid 12-month track record of stable levels can safely move to six-month intervals regardless of age. Your provider may also adjust the target itself: dropping to 0.40 to 0.60 mmol/L if you’re responding well but struggling with side effects, or pushing to 0.80 to 1.00 mmol/L if the current dose isn’t keeping symptoms in check.
Kidney Function
Lithium is cleared entirely by the kidneys, so monitoring renal health is non-negotiable. The preferred measure is the estimated glomerular filtration rate (eGFR), which gives a more accurate picture of how well your kidneys are filtering than older markers like creatinine alone. Guidelines vary on timing: the UK’s NICE recommends every six months, the British Association for Psychopharmacology says annually, and the American Psychiatric Association suggests every two to three months for the first six months, then every six to twelve months after that.
If your eGFR starts declining or drops below 60, your provider will likely add a urine protein test. A declining eGFR doesn’t necessarily mean you have to stop lithium, but it does mean closer surveillance and a conversation about whether the benefits still outweigh the risks.
Thyroid and Parathyroid Function
Lithium can slow down the thyroid, leading to hypothyroidism. Symptoms like fatigue, weight gain, and feeling cold can overlap with depression, which makes lab testing essential rather than relying on how you feel. TSH is the primary screening tool, typically checked every six to twelve months once you’re stable. Some providers also check antithyroid antibodies at baseline, since people who already have them are at higher risk for developing thyroid problems on lithium.
Less well known is lithium’s effect on the parathyroid glands, which regulate calcium. Lithium can cause these glands to become overactive, pushing calcium levels too high. Elevated calcium can cause bone loss, kidney stones, fatigue, and confusion. Monitoring serum calcium (and parathyroid hormone levels when calcium is elevated) every six months catches this before it becomes a problem.
Heart Rhythm
Lithium can affect the electrical conduction system of the heart. The most common ECG finding is T-wave flattening or inversion, seen in roughly 16 to 33 percent of people on lithium. The most common rhythm disturbance is a slow heart rate caused by the heart’s natural pacemaker being suppressed.
Other possible changes include a prolonged PR interval, first- or second-degree heart block, and, at toxic levels, dangerous QT prolongation that raises the risk of life-threatening arrhythmias. These changes tend to increase with age, particularly after 60. A baseline ECG followed by periodic repeat ECGs every 6 to 12 months is reasonable for most patients, with more frequent monitoring for anyone who has heart disease risk factors or unstable lithium levels.
Weight and Metabolic Health
Weight gain is a well-known concern with lithium, though recent data suggest the effect may be smaller than commonly assumed. Clinically significant weight gain is generally defined as an increase of more than 10 percent over your baseline body weight. Because this change can happen gradually, routine weight checks at every visit give you and your provider an early signal.
Tracking weight and metabolic markers (like blood sugar and cholesterol) from the very start of treatment helps separate lithium’s effects from conditions that were already developing. This is especially important because bipolar disorder itself is associated with higher baseline rates of obesity and metabolic syndrome.
Drug Interactions That Trigger Retesting
Several commonly prescribed medications can push lithium levels up, sometimes dangerously. The biggest offenders are thiazide diuretics (often prescribed for blood pressure), which can raise lithium concentrations by 25 to 40 percent. NSAIDs like ibuprofen and naproxen also reduce lithium clearance and have been linked to toxicity. ACE inhibitors, another common blood pressure class, may impair lithium elimination as well.
If you start or stop any of these medications, your lithium level should be rechecked promptly. The same applies to anything that changes your hydration status significantly: a new diuretic, a bout of vomiting or diarrhea, or a major dietary shift in salt intake. Dehydration concentrates lithium in the blood, and even a short illness can tip levels into the danger zone.
Recognizing Lithium Toxicity
Even with regular monitoring, knowing the warning signs of toxicity can prevent a medical emergency. Early toxicity, which can begin at levels above 1.5 mmol/L, typically affects coordination first: a coarse tremor, unsteady walking, and difficulty with fine motor tasks. As levels climb, you may notice slurred speech, muscle twitching, nausea, and blurred vision.
At levels persistently above 2.5 mmol/L, the risk of serious neurological injury rises sharply. Seizures, cerebellar damage (affecting balance and coordination permanently), and delirium are possible. Some of this damage can be irreversible, which is why catching toxicity early matters so much. If you develop a new or worsening tremor, confusion, or vomiting while on lithium, getting a stat blood level is the right move.
Monitoring During Pregnancy
Pregnancy changes how the kidneys handle lithium, so the monitoring schedule intensifies. The current recommendation is to check lithium levels roughly every three weeks through the first 34 weeks, then weekly from week 34 until delivery. In the first two weeks after birth, levels should be checked twice weekly, because the sudden drop in blood volume after delivery can cause lithium to spike.
Creatinine should be monitored alongside lithium levels throughout pregnancy. Any sign of preterm labor, pre-eclampsia, or illness affecting kidney function calls for even more frequent testing. The goal is to keep levels therapeutic while the body’s fluid balance shifts dramatically across trimesters and into the postpartum period.