Lithium can cause hyperthyroidism, but it’s uncommon. The drug is far more likely to slow your thyroid down than speed it up. Hypothyroidism affects roughly 6 to 52% of people on lithium therapy, depending on the study, while hyperthyroidism occurs in an estimated 0.5 to 10%. Still, that rate is higher than what’s seen in the general population, making it a real, if less frequent, side effect worth understanding.
Why Lithium Usually Slows the Thyroid
Lithium concentrates in thyroid tissue and interferes with the gland’s ability to produce and release its hormones. For most people, this means the thyroid becomes underactive. In one study of 150 patients on lithium, 19% developed mildly low thyroid function. Among those who already had thyroid antibodies in their blood (a sign of autoimmune activity), that number jumped to 53%. Women are more vulnerable than men. In a UK study of 274 people with mood disorders on long-term lithium, 17% of women developed hypothyroidism compared to just 3.5% of men.
So the typical story with lithium is a sluggish thyroid, not an overactive one. But the less common opposite reaction, hyperthyroidism, does happen, and it tends to follow a different pathway.
How Lithium Triggers Hyperthyroidism
The main route to lithium-related hyperthyroidism is a condition called silent thyroiditis, a type of painless inflammation of the thyroid gland. When the gland becomes inflamed, stored thyroid hormone leaks into the bloodstream all at once, producing a temporary surge. A study comparing 400 patients with thyroid overactivity found that those with silent thyroiditis were 4.7 times more likely to have been taking lithium than those whose overactive thyroid had other causes.
Lithium also appears to promote autoimmune activity against the thyroid. About 20% of people on lithium develop thyroid antibodies, compared to roughly 7.5% of people not taking it. This autoimmune stimulation can, in some cases, tip the thyroid into overproduction rather than underproduction.
The incidence of lithium-associated silent thyroiditis is approximately 1.3 cases per 1,000 person-years of use. For context, silent thyroiditis in the general population occurs at a rate below 0.28 per 1,000 person-years. Overall lithium-associated thyroid overactivity of any type runs at about 2.7 cases per 1,000 person-years, roughly double to triple the background rate.
When It Typically Appears
Most lithium-related thyroid problems show up within the first three years of treatment. One analysis found that among patients who eventually developed abnormal thyroid levels, over 90% did so within that window. Some people develop changes as early as the first three months. That said, thyroid dysfunction can emerge at any point during treatment, which is why ongoing monitoring matters for as long as you’re taking lithium.
Who Faces the Highest Risk
A large retrospective study published in The Lancet found that lithium significantly increased the risk of hypothyroidism (about 2.3 times the normal rate) but did not reach statistical significance for hyperthyroidism specifically. This reinforces that hyperthyroidism is the rarer outcome. The people most likely to develop any thyroid problem on lithium are women under 60 and those whose lithium blood levels run above the median therapeutic range.
Pre-existing thyroid antibodies also raise your risk. If your immune system is already primed against your thyroid before you start lithium, the drug’s effect on antibody production can push things over a clinical threshold in either direction.
What Lithium-Related Hyperthyroidism Feels Like
The symptoms mirror hyperthyroidism from any cause: a rapid or pounding heartbeat, unexplained weight loss, feeling hot when others are comfortable, tremor, anxiety, difficulty sleeping, and loose bowel movements. With silent thyroiditis specifically, there’s no pain or tenderness in the neck, which is what makes it “silent.” The overactive phase typically lasts weeks to a few months before the thyroid either returns to normal or swings into a period of underactivity.
Because people taking lithium often have bipolar disorder or other mood conditions, some of these symptoms (restlessness, poor sleep, anxiety) can be mistaken for a mood episode rather than a thyroid problem. This overlap makes blood testing essential rather than relying on symptoms alone.
How It’s Monitored
Current guidelines recommend checking thyroid function before starting lithium and then every 6 to 12 months for the first several years of treatment. If you develop symptoms or have risk factors like a family history of thyroid disease, your provider may test more frequently. The standard test is a TSH level, which rises when the thyroid is underactive and drops when it’s overactive. A low TSH reading prompts further testing of free thyroid hormone levels to confirm hyperthyroidism.
How It’s Managed
Treatment depends on the type of hyperthyroidism. Silent thyroiditis is often self-limiting, meaning the excess hormone phase resolves on its own as the stored hormone is depleted. During that phase, a beta blocker can help control symptoms like rapid heart rate and tremor.
If the overactivity is driven by autoimmune stimulation rather than simple inflammation, it may not resolve spontaneously. In those cases, the decision involves weighing the psychiatric benefits of continuing lithium against the thyroid consequences. Sometimes lithium can be continued while the thyroid problem is treated separately. In more resistant cases, definitive treatment such as radioactive iodine therapy or surgical removal of the thyroid may be considered.
Stopping lithium doesn’t always resolve thyroid overactivity immediately, and for people whose mood disorder is well controlled on lithium, discontinuation carries its own serious risks. These decisions are highly individual and involve balancing mental health stability with thyroid management.