Lithium does cause hypothyroidism in a significant number of people who take it. About 37% of lithium users develop some degree of thyroid dysfunction, though most cases are mild and temporary. Roughly 9% develop clinical hypothyroidism that requires treatment with thyroid hormone replacement.
How Lithium Affects the Thyroid
Lithium concentrates in the thyroid gland, where it interferes with several steps of normal thyroid hormone production. It blocks iodine uptake (the raw material your thyroid needs), disrupts how the gland assembles its hormones, and most importantly, inhibits the release of thyroid hormones into your bloodstream. That last effect, the blocked release, is the primary driver behind hypothyroidism and thyroid enlargement (goiter) in lithium users.
Lithium also affects the immune system in ways that can trigger thyroid problems. It ramps up the activity of certain immune cells while suppressing others, which can push susceptible people toward thyroid autoimmunity. In one study, 20% of lithium-treated patients had thyroid autoantibodies compared to 7.5% of people not taking lithium. These antibodies attack thyroid tissue and compound the direct hormonal effects, making hypothyroidism more likely and potentially more persistent.
How Common It Is
An Italian real-world study found that 36.7% of lithium-treated patients showed abnormal thyroid function at some point during treatment. For most, the changes were transient and resolved on their own without stopping lithium or starting any additional medication. Only 8.7% developed clinical hypothyroidism that needed thyroid hormone replacement.
A large study of 718 lithium patients reported a similar clinical hypothyroidism rate of 10.4%. The annual incidence was about 2.2% for women and 0.7% for men, meaning the risk accumulates the longer you stay on the medication. A Lancet analysis confirmed that lithium more than doubles the risk of hypothyroidism overall, with a hazard ratio of 2.31 after adjusting for age, sex, and diabetes.
When Thyroid Problems Typically Appear
Most lithium-related hypothyroidism shows up early. In a study tracking 42 cases, 38% were diagnosed within the first six months, 55% within the first year, and 74% within two years. That early window is why frequent thyroid monitoring matters most at the start of treatment. However, thyroid dysfunction can develop at any point during lithium use, so ongoing monitoring remains important even after the initial high-risk period.
The Lancet analysis also noted that adverse thyroid effects tended to occur early in treatment, and that higher lithium blood levels were associated with increased risk of all thyroid outcomes.
Who Is Most at Risk
Several factors make lithium-induced hypothyroidism significantly more likely:
- Sex. Women are affected far more often than men. Clinical hypothyroidism rates run about 14% in women versus 4.5% in men on lithium. Younger women (under 60) appear to be at particularly high risk.
- Age. Risk increases with age, especially for women over 50.
- Thyroid autoantibodies. People who already have antibodies against their own thyroid tissue are substantially more vulnerable. Among lithium users who test positive for these antibodies, the rate of subclinical hypothyroidism jumps to 53%.
- Family history. A family history of thyroid disease raises the likelihood of problems.
- Higher lithium levels. Blood lithium concentrations above the median are associated with greater risk of thyroid dysfunction.
The presence of thyroid autoantibodies also increases with age and duration of lithium treatment, and is more common in women. This creates a compounding effect where the people most biologically vulnerable are also the ones whose risk grows fastest over time.
Is It Reversible?
In many cases, yes. A retrospective study following 85 patients found that 41% were able to stop thyroid replacement therapy after discontinuing lithium. Of those who stopped, about a third did so within three months, and 71% were off thyroid medication within a year of quitting lithium. Only 17% of those who stopped thyroid replacement eventually needed to restart it.
That said, reversibility is not guaranteed. Some people develop permanent hypothyroidism, particularly those who had underlying thyroid autoimmunity that lithium unmasked or accelerated. For these individuals, the autoimmune process may continue even after lithium is out of the picture.
How It’s Monitored
Guidelines from the UK’s National Institute for Health and Care Excellence recommend thyroid function blood tests every six months for anyone on lithium. For people at higher risk (women over 50, those with thyroid antibodies, or those with a family history of thyroid disease), testing every three to four months is more appropriate. Real-world data suggests that many patients don’t actually get tested this often, which means thyroid problems can go undetected.
Thyroid function is typically checked with a simple blood test measuring TSH, the hormone your pituitary gland releases to tell your thyroid to work harder. When TSH rises above normal, it signals that your thyroid isn’t keeping up. Most clinicians check thyroid levels before starting lithium to establish a baseline, then follow up at regular intervals throughout treatment.
How It’s Treated
The standard approach is to add thyroid hormone replacement rather than stop lithium, since lithium is often essential for managing bipolar disorder or other mood conditions. If lithium is working well for your mental health, there’s rarely a reason to discontinue it just because of thyroid changes.
The choice of thyroid hormone replacement can depend on your specific diagnosis. For people with bipolar disorder, T4 (the standard thyroid replacement) is generally preferred because it may also help stabilize mood. For people with unipolar depression, T3 (a more active form of thyroid hormone) may offer the added benefit of boosting antidepressant response. In either case, careful dose adjustments are important to avoid overcorrecting into hyperthyroidism, which can destabilize mood.
For people with mild TSH elevations and no symptoms, treatment may not be necessary right away. Some clinicians take a watch-and-wait approach, since transient thyroid fluctuations are common and often resolve on their own without any intervention.