Hypothyroidism is the disease state in humans and animals caused by insufficient production of thyroid hormone by the thyroid gland....

Hypothyroidism is the disease state in humans and animals caused by insufficient production of thyroid hormone by the thyroid gland.

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

Secondary hypothyroidism:  origin – 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: origin -  hypothalamus
Results when the hypothalamus fails to produce sufficient TRH (thyroid releasing horomone).


Hyperthyroid
Hyperthyroidism (or "overactive thyroid gland") is the clinical syndrome caused by an excess of circulating free thyroxine (T4) or free triiodothyronine (T3), or both.  Thyroid hormone generally controls the pace of all of the processes in the body. This pace is called one's metabolism. If there is too much thyroid hormone, every function of the body tends to speed up. The thyroid gland regulates the body temperature by secreting two hormones that control how quickly the body burns calories and energy. If the thyroid produces too much hormone, the condition is called hyperthyroidism, but if too little is produced, the result is hypothyroidism.
Major causes in humans are:
•    Graves' disease (the most common etiology with 70-80%)
•    Toxic thyroid adenoma
•    Toxic multinodular goitre
•    Postpartum thyroiditis