Goiter in simple words means swollen thyroid. The common perception among community is any thyroid related disorders means abnormal functional thyroid, however it is important to understand that the presence of a goiter does not necessarily means functional thyroid abnormality.
A goiter can occur in a overactive (Hyperthyroid), underactive (Hypothyroid) or normal active (Euthyroid) gland.

One of the important reasons of goiter development is iodine deficiency. When there is not enough iodine, thyroid gland become underactive and send signal to brain to secret more TSH which in turn stimulate thyroid to produce more hormone and to grow in size. This abnormal growth is known as goiter.Other cause of goiter includes Hashimotos thyroiditis, Graves disease, or thyroid cancer.
Serum TSH, Ultrasound of neck or Thyroid scan and Fine needle aspiration cytology is usually done to make the definitive diagnosis. If the goiter size is small, not causing any compressive symptoms (like difficulty in breathing, swallowing, voice change or cough on lying down) and is non malignant it can be managed conservatively otherwise surgical intervention is warranted.