Thyroid gland

The thyroid gland is located in the throat. It is part of the endocrine system being controlled by the Thyroid Stimulating Hormone (TSH) being one of the largest endocrine glands of the body,the thyroid gland has two distinctive roles: 

1. secretion of the thyroid hormone which helps in maintainance of metabolism in tissues also stimulating oxygen consumption by the body
2. seceretion of calcitonin which helps in calcium circulation.

The thyroid hormone isnt needed  essentially for life processes but its absense or dysfunctioning can have ill effects on the body depending on the age group of the person e.g. these hormones play an important role in fetal and neonatal stages as its absense here can result in severe mental retardation or in somecses even dwarfism.

In adults to hypo and hyper thyroidism have distinct symptoms,a person suffering from hyperthyroidism would have a very fast metabolic rate and would lose weight very fast in comparison to a person with hypothyroidism who would gain wait very fast ^[1].

References

1. ↑ Barrett,Barman,Boitano,Brooks (2010). GANGONGS REVIEW OF MEDICAL PHYSIOLOGY. UNITED STATES OF AMERICA: McGRAW Hill companies. 301-302.