Thyroid gland

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

Thyrotoxicosis
Thyrotoxicosis is a state of excess thyroid hormones in the circulation resulting in signs and symptoms of thyroid over-activity.

Hyperthyroidism
Hyperthyroidism is the term for overactive tissue within the thyroid gland, resulting in overproduction and thus an excess of circulating free thyroid hormones: thyroxine (T4), triiodothyronine (T3), or both. Thyroid hormone is important at a cellular level, affecting nearly every type of tissue in the body.

Thyroid hormone functions as a stimulus to metabolism and is critical to normal function of the cell. In excess, it both overstimulates metabolism and exacerbates the effect of the sympathetic nervous system, causing "speeding up" of various body systems and symptoms resembling an overdose of epinephrine (adrenaline). These include fast heart beat and symptoms of palpitations, nervous system tremor and anxiety symptoms, digestive system hypermotility (diarrhea), and weight loss.

Cells of the thyroid
sources:

http://en.wikipedia.org/wiki/Hypothyroidism

http://en.wikipedia.org/wiki/Hyperthyroidism