Chemical Coordination and Integration - Notes | Class 11 | Pat 2: Thyroid and Parathyroid



Largest endocrine gland.

It includes 2 lobes on either side of the trachea. 

The lobes are interconnected with isthmus (a connective tissue).

Thyroid gland is made of follicles & stromal tissues.

Follicular cells secrete the following hormones:

1. Thyroxin (tetraiodothyronine, T4) & Triiodothyronine (T3): 

Their functions are
  • Regulation of basal metabolic rate (BMR).
  • Physical, mental and sexual development.
  • Support RBC formation.
  • Control metabolism of carbohydrates, proteins & fats.
  • Maintain water and electrolyte balance.
2. Thyrocalcitonin (TCT): 

A protein hormone. It regulates (lowers) blood calcium levels (Hypocalcaemic hormone).

Iodine is essential for normal hormone synthesis in thyroid.

Hypothyroidism (Goiter):
  • Enlargement of thyroid gland due to deficiency of iodine.
  • In adult women, it causes irregular menstrual cycle.
  • Hypothyroidism during pregnancy affects the baby causing stunted growth (cretinism), mental retardation, low intelligence quotient, abnormal skin, deaf-mutism etc.
  • Abnormal increase of thyroid hormones resulting in adverse effects on the physiological activities.
  • It is caused due to development of the nodules or the cancer of thyroid gland.
  • Exophthalmic goiter (Grave’s disease): It is a form of Hyperthyroidism. Symptoms are enlargement of thyroid gland, protruded eyeballs, increased BMR & weight loss.


4 parathyroid glands are present on back side of the thyroid gland, one pair each in the two lobes of thyroid gland. 

They secrete Parathyroid hormone (PTH) – a peptide hormone.

Functions of parathyroid hormone:
  • Increases Ca2+ level in blood (hypercalcaemic hormone).
  • Stimulates the bone resorption (demineralization).
  • Stimulates the reabsorption of Ca2+ by the renal tubules and increases Ca2+ absorption from the digested food.
  • Along with TCT, it helps in calcium balance in the body.


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