What are the causes of hypothyroidism?

There are several distinct causes for chronic hypothyroidism.

Iodine

Historically and, still, in many developing countries iodine deficiency is the most common cause of hypothyroidism world-wide. In present day developed countries, however, hypothyroidism is mostly caused by Hashimoto's thyroiditis, or by a lack of the thyroid gland or a deficiency of hormones from either the hypothalamus or the pituitary.

Hypothyroidism in dogs

Hypothyroidism can also result from sporadic inheritance, sometimes autosomal recessive. It is a relatively common disease in purebred domestic dogs as well, and can have a hereditary basis in dogs.

Temporary hypothyroidism can be due to the Wolff-Chaikoff effect.
Hypothyroidism is often classified by the organ of origin:

Primary hypothyroidism (thyroid gland)
The most common forms include Hashimoto's thyroiditis (an autoimmune disease) and radioiodine therapy for hyperthyroidism.

Secondary hypothyroidism (pituitary gland)
Occurs if the pituitary gland does not create enough thyroid stimulating hormone (TSH) to induce the thyroid gland to create a sufficient quantity of thyroxine. Although not every case of secondary hypothyroidism has a clear-cut cause, it is usually caused when the pituitary is damaged by a tumor, radiation, or surgery so that it is no longer able to instruct the thyroid to make enough hormone.

Tertiary hypothyroidism, also called hypothalamic-pituitary-axis hypothyroidism (hypothalamus)
Results when the hypothalamus fails to produce sufficient TRH.

General psychological associations

Hypothyroidism can be caused by lithium-based mood stabilizers used to treat bipolar disorder (previously known as manic depression).
In addition, patients with hypothyroidism and psychiatric symptoms may be diagnosed with:

atypical depression (which may present as dysthymia)
bipolar spectrum syndrome (including bipolar I or bipolar II disorder, cyclothymia, or premenstrual syndrome)
anxiety
inattentive ADHD or sluggish cognitive tempo