What lab tests need to be checked when someone is taking lithium for bipolar disorder?

Periodic blood work is a fact of life with lithium, as with many other medications.

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Prior to starting lithium, it’s typical to check the following:

Electrolytes (sodium, potassium, chloride, bicarbonate)
Thyroid function tests
Measures of renal function (blood urea nitrogen/creatinine, aka BUN/CR)
A complete blood count
Pregnancy test in women of child-bearing age (Lithium can in a very small number of cases cause birth defects, most notably a cardiac malformation called Ebstein’s anomaly, especially during the first-trimester. Women should be warned of this rare but potentially adverse reaction, and if sexually active, birth control is recommended.)

Lithium levels, This is a blood test that should be performed a few days (usually five on an outpatient basis, but sometimes more frequently on an inpatient) after the person starts taking the medication, and a few days after a change in dosage. The reason that a few days are needed is so that the medication is at a steady state in the body, where what you’re taking in on a daily basis is roughly equivalent to the amount of drug being excreted through the kidneys. A lithium level is tested in the morning before taking your dose (a trough level). When on a regular dosage, people typically have their lithium levels checked at least twice a year, more frequently in older people or those experiencing side effects.
Thyroid function tests, Lithium can damage the thyroid gland and interfere with the production of thyroid hormone. Regular blood tests, twice a year, should be done to monitor this.
Renal function tests, over time, lithium can cause a decrease in the rate at which the kidneys function to filter waste products from the body. Twice yearly tests of blood urea nitrogen (BUN) and creatinine, measures of kidney health, should be done, more frequently if there is evidence of declining kidney function.