An underactive thyroid and depression can both affect a person’s mood during their daily life. There is also increasing evidence of links between these two conditions.
Hypothyroidism is a condition where the thyroid produces too few hormones. Possible causes of the condition include autoimmune disease, removal of the thyroid, or radiation treatment.
The thyroid is a small gland in the neck, just in front of the throat. The gland produces hormones that are essential for converting food into energy and supporting growth.
Problems with the thyroid can have many consequences, including causing symptoms of depression.
There may be some links between these two conditions, but it is unlikely that depression causes hypothyroidism.
There is research, however, to suggest that hypothyroidism may lead to depression in some people.
It is also possible that medications for mood disorders that interfere with people’s mental health can affect their thyroid functioning.
Lithium is a common treatment or bipolar disorder.
Lithium affects the thyroid and prevents the production of T3 and T4 hormones. People taking lithium are more likely to develop hypothyroidism.
However, some people with bipolar disorder are more likely to have hypothyroidism regardless of lithium treatment.
An older study from 2002 found that 28% of participants with bipolar disorder also had hypothyroidism. The condition was present in only 3% of a general population control and 18% of participants with other psychiatric disorders.
SSRIs can reduce T4 hormone levels by 11.2%. They also affect hormone signaling from the hypothalamus region of the brain. This limits hormone production even further.
One 2015 study found that short-term SSRI use may cause hypothyroidism in very rare cases. It found that when switching to a different medication (venlafaxine), the thyroid returned to normal after 5 months.
When the thyroid is not working as it should, some people develop mental health problems. If the thyroid produces too many or too few hormones, it can cause mood changes.
Hypothyroidism can affect the ability to think. This can make it difficult to learn new things and may affect mood.
People with severe hypothyroidism may experience symptoms of depression or dementia, what some term pseudodementia.
Thyroid is a hormone that affects some of these symptoms, although, it is still unclear how it influences the brain. However, changes in thyroid levels can increase the risk of mental health disorders.
Hormone changes in the hypothalamic pituitary thyroid (HPT) axis can also lead to depression. Even in people with normal thyroid levels, hormone changes in the HPT axis can cause an episode of depression.
These hormone changes interrupt the movement of chemical signals in the brain. They mostly affect the hippocampus, a part of the brain important for learning and memory.
The changes can cause symptoms that include:
- behavioral changes
- difficulty learning new things
- memory problems
- speaking difficulties
Hypothyroidism has many symptoms. The earliest symptoms are psychiatric, affecting how the brain works.
These symptoms account for 2–12% of early symptoms in people with hypothyroidism. They include:
- memory lapses
- difficulty concentrating
- speech problems
Around 30–40% of people with hypothyroidism also have an anxiety disorder. However, it remains unclear whether hypothyroidism causes the anxiety disorder.
Some studies have found that about as many people have hypothyroidism and anxiety as anxiety alone. This suggests that some people are more likely to experience both as a result of other factors, such as genetics or lifestyle.
People with hypothyroidism and anxiety may respond to levothyroxine (L-T4) therapy. This therapy replaces the hormones that hypothyroidism reduces.
But L-T4 therapy is not always useful for anxiety symptoms. In one study, the use of L-T4 therapy worsened anxiety symptoms.
Hypothyroidism and depression can produce similar symptoms. For example, both can cause changes in mood.
There may be a link between hypothyroidism and depression. It is more likely that hypothyroidism causes symptoms of depression, rather than depression leading to hypothyroidism.
For example, hypothyroidism can affect hormone production and disrupt chemical signaling in the brain, leading to depression.
There is a link between hypothyroidism and the use of some medications for depression and bipolar disorder. For example, lithium affects hormone production by the thyroid, which might lead to hypothyroidism.
Some SSRIs also affect thyroid production. But changing to a different medication may reverse hypothyroidism.
Hypothyroidism may also cause anxiety symptoms, although, the evidence for this is weak. L-T4 therapy for hypothyroidism and anxiety could reduce symptoms in some people but might worsen them in others.