he thyroid is a butterfly-shaped endocrine (hormone secreting) gland in the neck that is found on both sides of the trachea (windpipe). It secretes the hormone Thyroxine which controls the rate of metabolism.
The thyroid weighs about 20-25 gms in an adult. It is butterfly shaped and it consists of two lobes which are connected together by a median isthmus as shown in the picture. It is located in the front of the neck and is situated just below the larynx or Adam's apple.
The function of the thyroid gland is to take up iodine from the foods that we eat and to convert it into thyroid hormones namely Thyroxine (T4) and Triiodothyronine (T3) and Calcitonin (which is involved in calcium metabolism).
The production of thyroid hormones is under the direct control of the pituitary gland. The pituitary gland is an endocrine gland which is situated at the base of the brain and it secretes certain vital hormones which include TSH (Thyroid stimulating hormone) or Thyrotropin, besides others.
Diagnosis of hypothyroidism can be established based on the results of tests done to measure the levels of these hormones in the blood. Commonly T4 and TSH in blood are measured. Since hypothyroidism is about deficiency or low levels of thyroid hormones, T4 (and T3) levels in the blood are below normal.
TSH levels are high in case of Primary hypothyroidism and this can be explained as follows. Since the thyroid is not able to produce adequate amounts of T4 and T3, the pituitary senses this and increases TSH (Thyroid stimulating hormone) production which stimulates the thyroid to increase T4and T3 production.
In case of secondary hypothyroidism, the pituitary gland itself fails to produce enough TSH and hence the levels of TSH as well asT4 and T3 will be below normal in the blood.