Hypothyroidism isn’t a direct cause of bipolar disorder, but research indicates that thyroid dysfunction may contribute to its progression and increase the severity of symptoms.

Some people with bipolar disorder experience coexisting conditions, including hypothyroidism, also known as underactive thyroid.

A 2024 research review showed that people with bipolar disorder had a 2.5-fold higher chance of having atypical thyroid hormone levels than the larger population.

No evidence points to hypothyroidism being a direct cause of bipolar disorder.

However, some research shows that an underactive thyroid may contribute to the progression of certain types of bipolar disorder, especially if someone is predisposed to the condition. Thyroid function may also affect symptom severity.

Changes in the hypothalamic-pituitary-thyroxine (HPT) axis are common in bipolar disorder. The HPT axis regulates many body functions, including hormone and neurotransmitter levels associated with mood and thyroid function.

Research on this link is inconclusive so far, but a 2021 study suggested thyroid hormone levels in people with bipolar disorder may change with mood episodes. The study participants experiencing manic episodes (periods of elevated mood) seemed to have higher levels of thyroid hormones than those experiencing depressive episodes.

But a small 2019 study contradicts those findings. The study found no evidence of significant changes in thyroid hormone levels between mood episodes in bipolar disorder.

A 2023 study, which examined the first episodes of bipolar disorder in females, revealed clinical signs of thyroid dysfunction in 27% of participants going through a manic or hypomanic episode and 60% experiencing depressive episodes.

Specifically, researchers found that triiodothyronine (T3) hormone tests showed lower hormone levels in people experiencing a first episode of bipolar depression than in those experiencing mania or hypomania for the first time. Lower T3 levels also linked with more severe symptoms.

Researchers concluded that the findings may show that thyroid dysfunction disrupts mood and influences bipolar disorder progression in people already predisposed to the condition.

More research with various populations and larger sample sizes is necessary to further understand this relationship’s underlying mechanisms.

The lithium-hypothyroidism link

If you have bipolar disorder and currently take the mood stabilizer medication lithium, you may have a higher chance of having an underactive thyroid. This higher chance may be because lithium disrupts thyroid function, as a 2019 research review indicated.

While the exact mechanism isn’t clear, experts believe lithium inhibits the release of thyroid hormones T3 and T4 (thyroxine).

A different 2023 study found that 32% of the participants receiving long-term lithium treatment for bipolar disorder developed a thyroid disorder during treatment. Of those, 79% received a hypothyroidism diagnosis.

Ultimately, the relationship between hypothyroidism and bipolar disorder is complex and misunderstood. It involves factors like the role of thyroid hormones on mood, the bipolar disorder type, the underlying causes of hypothyroidism, and the treatment type.

As mentioned, an underactive thyroid may affect the HPT axis, which can change neurotransmitters and hormones that influence mood and cognition (how you think).

A research review from 2023 shared that while hypothyroidism could contribute to the onset of mania and bipolar depression symptoms, it often links with depressive symptoms.

Possible mental health symptoms of hypothyroidism include:

  • low mood (hopelessness, loneliness, emptiness)
  • persistent sadness
  • low motivation
  • inability to experience pleasure and joy
  • anxiety episodes
  • difficulty with concentration and memory
  • persistent mood changes
  • irritability
  • social withdrawal

Hashimoto thyroiditis, also known as Hashimoto disease, is an autoimmune condition that causes thyroid gland inflammation and damage.

As the most common cause of hypothyroidism, Hashimoto can manifest with mental health symptoms similar to those in bipolar disorder, including mood changes.

One different study from 2019 suggested that elevated levels of anti-thyroid antibodies in conditions like Hashimoto thyroiditis may link with rapid cycling in bipolar disorder when both disorders occur together.

The link between hypothyroidism and bipolar disorders is complex and misunderstood. Experts theorize thyroid hormone dysfunction could contribute to mood changes that influence bipolar disorder progression in people already predisposed to the condition.

Bipolar disorder treatment with lithium may also increase the chance of developing hypothyroidism by reducing your thyroid gland function.

More research is necessary to determine whether and how thyroid function affects bipolar disorder development and progression and vice versa.