THYROID AND HYPOTHYROIDISM

 

The thyroid gland is one of the endocrine glands, located immediately below the larynx on either side of and anterior to the trachea.

 

Physiological Anatomy of Thyroid

The thyroid gland is composed of large number of closed follicles filled with a secretory substance called colloid and lined with cuboidal epitheliod cells that secrete into the interior of the follicles. The major constitutent of colloid is the large glycoprotein known as thyroglobulin, which contains the thyroid hormone.

                                     

Normal Weight:

about 25 grams.

 

Thyroid Hormones:

1. Thyroxine or Tetraiodothryronine or T4

2. Triiodothyronine or T3

3. Calcitonin: an important hormone for calcium metabolism

 

 

Formation and Secretion of T3 and T4

Iodine requirement:  is about 1 mg per week of ingested iodine is required        to form normal quantities of thyroxine. To prevent iodine deficiency common table salt is iodized with one part of sodium iodide to every 100,000 parts of sodium chloride.

 

Steps of T3 and T4 formation:

 

Iodine + Tyrosine*                             Monoiodotyrosine

                                                Diiodotyrosine

 

Monoiodotyrosine + Diiodotyrosine              Triidothyronine (T3)

 

Diiodotyrosine + Diiodotyrosine                    Tetraiodothyronine or Thyroxine (T4)

         

                  

Iodine 

in Diet
 

Iodine + Tyrosin
 
 

 

 


Blood
 
                   Absorption facilitated                                                    Monoiodotyrosin                  T3

                    By TSH                                                                  Diiodotyrosine                  T4

                                                                                                              +

Bowel
 
                                                                                                      Diiodotyrosine

Thyroid Gland
 
 


                                                                                             Secretion stimulated by TSH

 

 

 

HYPOTHALAMUS
 

TRH
 

T3
 

T4
 

TSH
 
Text Box: +Text Box: +

 

 

 

 

 

 

 

 

 


PITUITARY
 
 

 

 

 

 

 

 

 

 

 

 


 

 

 

TYROSIN + IODINE
 

THYROID
 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

=
 

HYPOTHYROID
 

HYPERTHYROID
 

SUBCLINICAL

HYPOTHYROIDISM
 

=
 
 

 

 

 

 

 

 

 

 

 



Regulation of Thyroid Hormone Secretion

 

The rate of thyroid hormone secretion is controlled by a specific feedback mechanism which operates through the hypothalamus and anterior pituitary gland. This system can be explained as follows:

 

1.     Effects of Thyroid Stimulating Hormone (TSH) on Thyroid Secretion: TSH is secreted by the Anterior Pituitary. It increases the secretion of thyroxin and T3 by the thyroid gland.

2.     Hypothalamic Regulation of TSH: Hypothalamus secretes a thyrotropin releasing hormone (TRH) which increases the secretion of TSH by anterior pituitary and correspondingly increases the activity of the thyroid gland. Hypothalamus can also inhibit the secretion of TSH by secreting a hormone somatostatin.

3.     Feedback regulation of Thyroid secretion: Increased levels of thyroid hormone in the body fluids decreases the secretion of TSH by the anterior pituitary, which ultimately leads to decreased secretion of thyroid hormone from the thyroid gland.

 

 

HYPOTHYROIDISM

 

Definition:

This is a clinical condition resulting from deficient thyroid hormone secretion.

Types:

 

 

1.     Cretinism or Congenital hypothyroidism: Hypothyroidism dating from birth and resulting in developmental abnormalities in known as cretinism. It my occur due to thyroid agenesis, ectopic or hypoplastic thyroid tissue, inherited disorders or hormonogenesis, and transplacental passage of thyroid stimulating hormone receptor blocking antibodies (such cases resolve spontaneously within 2 months)

2.     Myxoedema: It is characterized by accumulation of hydrophilic mucopolysaccharides in the ground substance of the skin and other tissues leading to thickening of the facial features and doughy indurations of the skin.

 

 

Causes:

I:   Secondary to Pituitary / Hypothalamic disease

 

II:  Primary thyroidal causes:

  1. Inhibition of synthesis of thyroid hormone:

(i)                Iodine deficiency

(ii)             Antithyroid drugs

(iii)           Inherited enzyme defects

 
  1. Destruction of Gland:

(i)                Surgical Removal

(ii)             Irradiation (radioactive iodine, external)

(iii)           Autoimmune Disease (Hashimoto’s thyroiditis)

(iv)           After thyroiditis (acute and subacute)

(v)              Replacement by cancer or other diseases.

 
  1. Agenesis
  2. Idiopathic atrophy (related to Hashimoto’s)

Presentation of Hypothyrodism

 

 

 

Rheumotology

Aches and pains, arthritis

Gynaecology

Menorrhagia

Ophthalmologist

Puffy Eyes

ENT

Deafness, Hoarse Voice

Neurology

Polyneuritis, Cerebellar Features

Dermatology

Dry Skin, Alopecia

Psychiatry

Mental Changes, Psychosis

Cardiologist

Ischemia, Cardiac Failure

Gastroenterology

Constipation

Surgery

Carpal-tunnel Syndrome, Goiter

Hematology

Anemia

Casualty

Coma

Nephrology

Puffy Face, Oedema

Fortuitous presentation

Screening or routine biochemical profile

 

 

The general physician may see any of the above presenting features, but should consider hypothyroidism in any patient whose mental or physical or general health has changed without explanation.

 

CLINICAL FEATURES

 

CRETINISM

 

IN INFANTS

 

1. Persistence of physiological jaundice

2. Hoarse cry

3. Motionless body

4. Noisy breathing

5. Constipation

6. Sleeps all day

7. Skin feels cold and dry

8. Pallor

9. Puffy face

10. Feeding problem

 

 

IN CHILDREN

 

1. Short stature

2. Course features

3. Widely set eyes

4. Broad flat nose

5. Protruding tongue

6. Sparse hairs

7. Cool dry skin

8. Thick neck

9. Protrubent abdomen with umbilical hernia

10. Delayed and defective dentition

 

 

IN OLDER CHILDREN

 

 

1. Mental retardation

 

2. Delayed puberty

 

3. Prolonged tendon reflex relaxa