The short answer: Lithium works in a narrow band of blood levels, and the toxic level sits close to the helpful one. The FDA label for lithium carbonate puts the therapeutic range at 0.8 to 1.2 mEq/L and toxic concentrations at 1.5 mEq/L or higher. A blood test is the only way to see where you are. Other tests watch your kidneys, thyroid, and calcium, because long-term lithium can affect all three.

What do lithium blood tests check?

A lithium monitoring visit usually covers four things. Each test answers a different question.

  • Serum lithium level. This shows how much lithium is in your blood. It tells your prescriber if the dose is too low to work or high enough to cause harm.
  • Kidney function. The label states that lithium is substantially excreted by the kidneys. If the kidneys slow down, lithium builds up. The label says kidney function should be assessed before and during treatment, with tests such as serum creatinine. The NICE bipolar guideline (CG185) also lists urea and electrolytes and an estimated glomerular filtration rate (eGFR).
  • Thyroid function. The label explains that lithium is concentrated within the thyroid and can lead to hypothyroidism (an underactive thyroid). It also reports some cases of an overactive thyroid.
  • Calcium. The label links long-term lithium treatment with persistent hyperparathyroidism and high blood calcium. It tells clinicians to monitor serum calcium regularly.

How big are these risks? A 2012 meta-analysis in The Lancet pooled 385 studies. Clinical hypothyroidism was more common with lithium than with placebo (odds ratio 5.78). Blood calcium rose by 0.09 mmol/L on average, and parathyroid hormone rose by 7.32 pg/mL. The ability of the kidneys to concentrate urine fell by 15% of the normal maximum.

The same analysis is reassuring about severe kidney damage. The authors found little evidence of a clinically significant drop in kidney function in most patients. Only 18 of 3,369 patients (0.5%) needed renal replacement therapy, such as dialysis. Regular tests exist to find the few people who are affected, early. Our sibling site thyroid.md explains thyroid tests in more detail.

Why is the safe range so narrow?

The gap between a helpful level and a harmful level is small. The label carries a boxed warning, the strongest type the FDA uses. It says lithium toxicity is closely related to serum lithium concentrations and can occur at doses close to therapeutic concentrations.

The numbers show how tight the window is. The label gives a goal of 0.8 to 1.2 mEq/L for acute mania and 0.8 to 1.0 mEq/L for maintenance. It lists toxic concentrations as 1.5 mEq/L or higher.

Targets differ between sources. NICE aims for 0.6 to 0.8 mmol/L in people prescribed lithium for the first time. It suggests considering 0.8 to 1.0 mmol/L for at least 6 months in people who relapsed on lithium before. An international task force (ISBD/IGSLI) reached consensus on 0.60 to 0.80 mmol/L as the standard maintenance level for adults. For lithium, mEq/L and mmol/L are the same number.

A level inside the range is not a guarantee. The label warns that some people show toxic signs at concentrations within the therapeutic range. It adds that older adults may show toxicity at levels other patients tolerate. Your prescriber sets your personal target. Do not change your dose based on a lab result alone.

When should the blood be drawn?

Timing changes the result. The level rises after each dose and then falls, so a sample taken too early reads high.

The label says blood should be drawn immediately before the next dose, when concentrations are relatively stable. It defines this as 12 hours after the previous dose. The published target ranges assume that timing.

Ask your prescriber or the lab how to time your dose on a test day. Tell the person who draws your blood when you took your last dose. A result with the wrong timing can lead to a wrong dose decision.

How often are the tests repeated?

Tests are most frequent at the start. They become less frequent once your level is stable.

At the start. The label calls for a first lithium level after 3 days, then regular levels until you are stabilized. NICE recommends a level 1 week after starting and 1 week after every dose change, then weekly until levels are stable. Both sources call for kidney and thyroid tests before the first dose. NICE adds calcium and a full blood count.

During long-term treatment. NICE recommends a lithium level every 3 months for the first year. After that, it recommends every 6 months for most people. It keeps the 3-month interval for some groups:

  • older people
  • people who take drugs that interact with lithium
  • people at risk of impaired kidney or thyroid function or raised calcium
  • people with poor symptom control or poor adherence
  • people whose last level was 0.8 mmol/L or higher

NICE also recommends kidney, calcium, and thyroid tests every 6 months. The FDA label asks for thyroid tests before treatment, at three months, and every six to twelve months after that. It says calcium and kidney function should be monitored but gives no fixed interval.

NICE is a United Kingdom guideline, and US practice varies. Your own schedule may differ. For background on the drug itself, read lithium: what to know.

What can push your lithium level up?

Your level can rise even when your dose stays the same. The label lists these risks for toxicity:

  • Dehydration or fluid loss. The label names volume depletion, protracted sweating, diarrhea, and illness with fever.
  • NSAIDs. The label says these pain relievers decrease renal blood flow, which lowers lithium clearance and raises the level. NICE tells clinicians to warn people on lithium not to take over-the-counter NSAIDs.
  • Diuretics. The label says the sodium loss they cause may reduce lithium clearance.
  • Renin-angiotensin system antagonists. This is the label's term for the blood pressure drug group that includes ACE inhibitors. The label says they increase steady-state lithium concentrations.
  • Metronidazole. The label lists this antibiotic type as one that may raise lithium levels.
  • Changes in salt intake. The label says it is essential to keep a normal diet, including salt, and an adequate fluid intake.

The label calls for a lithium level after any change in dose or other medicines, and during another illness. It also names a marked change in strenuous exercise. Tell every doctor and pharmacist that you take lithium before you start a new medicine. Our mood stabilizers guide compares monitoring needs across drugs.

What are the warning signs of lithium toxicity?

Know the signs, because toxicity can develop between tests. MedlinePlus says to stop taking lithium and call your doctor immediately if you have any of these:

  • drowsiness
  • shaking of a part of your body that you cannot control
  • muscle weakness, stiffness, twitching, or tightness
  • loss of coordination
  • diarrhea or vomiting
  • slurred speech
  • giddiness
  • ringing in the ears
  • blurred vision

The label describes severe toxicity as confusion, seizure, coma, and death. It states that no specific antidote for lithium poisoning is known. It also notes that symptoms can be delayed, because lithium may take up to 24 hours to reach brain tissue.

For a suspected overdose, MedlinePlus gives the poison control helpline, 1-800-222-1222. Call 911 if the person has collapsed, had a seizure, has trouble breathing, or cannot be woken.

A pause for suspected toxicity is a decision to make with your prescriber. It is different from quitting. Stopping lithium for good has its own risks, which we cover in what happens if you stop taking lithium.

The bottom line

Lithium needs regular blood tests because the helpful level and the harmful level are close together. The lithium level checks your dose. Kidney, thyroid, and calcium tests find slow changes early, while they can still be managed. Keep the 12-hour timing, keep your appointments, and report dehydration, new medicines, and toxicity signs quickly.

Last updated: October 2026. This article is for informational purposes only and does not constitute medical advice. Ask your own prescriber which lithium level, test schedule, and dose apply to you, and never change your dose on your own.