Thyroid:

It is the only endocrine gland whose product is stored in large quantity.

Origin: From endoderm of embryo.

Position: Lies of front of Larynx (voice box).

Hormones: It secrets 3 Hormones:

1. Thyroxine (T4)1 or (Tetraidothyronine)	2. Triidothyronine (T3)2	3. Calcitonin
1. It has 4 iodine atoms.	1. It has 3 iodine atoms.	1. It has no iodine atom.
2. It controls the functioning of kidney. Its deficiency decreases urine output.	2. It does not.	2. It also does not.
3. It does not.	3. It does not.	3. It regulates the concentration of calcium and phosphorus in blood. So, it is also called hypo-calcemic hormone.

  

Similar functions of (T₄)¹and (T₃)²:

1.       Both tetraidothyronine and triidothyronine controls (BMR) basal metabolic rate of body.

2.       Both promote body growth, and mental development.

3.       Both are iodinated forms of amino acid tyrosine. They are stored in follicles & released to blood when needed. (Stored amount is enough for 10 months.

Disorders caused by improper secretion of Thyroid hormone:

1.Cretinism: Caused by low secretion of thyroid hormone in infants. It reduces metabolic rate and oxidation. And leads to, slow heart beat, low BP, low temperature of body, stops growth and causes mental retardation.

2.Iodine deficiency (Goitre): Caused by deficiency of iodine in diet because iodine is needed for the synthesis of thyroid hormones. It causes enlargement of thyroid gland which leads to pain. This further of two types:-

i)Iodine deficiency Goiter:- Hypo-secretion of thyroxin 

ii)Exopthalmic Goiter:- Hyper-secretion of thyroxin