Thyroid Gland - Hormones and their functions

Thyroid gland is a bilobed, butterfly-shaped organ which is the largest endocrine gland in the body. The medical term for thyroid gland is "glandula thyreoidea". It secretes three hormones: T3 hormone, T4 hormone and Calcitonin. 

• Weight: 20-60 grams
• Size: 2-inches long

Wharton described the Thyroid gland for the first time.

Topics Covered:

1) Location of Thyroid gland

2) Structure of Thyroid gland

    A. Follicular cells

    B. Parafollicular cells

3) Functions of Thyroid gland

4) Hormones secreted by Thyroid gland

    A.  T3 and T4 hormones

• Secretion
• Iodine - Building block of T3 and T4
• Regulation - Negative Feedback Mechanism
• Functions
• Hypothyroidism - Simple goitre, Cretinism, Myxoedema
• Hyperthyroidism - Exophthalmic goitre

    B. Calcitonin

• Secretion
• Function
• Calcitonin vs Parathormone

1) Location of Thyroid Gland:

Thyroid gland is found in front of neck, under the larynx (voice box).

The two lobes of Thyroid gland lie on either side of trachea (windpipe) and are interconnected through a narrow strip of tissue called isthmus.

2) Structure of Thyroid Gland

Thyroid gland consists of two types of cells:

• Follicular cells
• Parafollicular cells or C-cells

A. Follicular cells

Thyroid gland consists of numerous oval-shaped, sac like structure called as follicle, which secretes thyroid hormones (T3 and T4).

Each follicle is lined by epithelial cells and contains a jelly-like substance called colloid. Colloid contains precursor of thyroid hormones.

i) When the activity of thyroid gland is low to moderate,

• shape of epithelial cell is cuboidal 
• follicle contains large quantities of colloid

ii) When the activity of thyroid gland is high,

• shape of epithelial cell is columnar
• follicle contains lower quantities of colloid

B. Parafollicular cells

Parafollicular cells or C-cells are present adjacent to follicular cells and resides in the connective tissue.  C-cells are responsible for the secretion of Calcitonin hormone.

3)  Functions of Thyroid gland:

Thyroid gland controls 

• basal metabolism rate of the body (T3 and T4) and 
• calcium levels in the bone (Calcitonin)

4)  Hormones Secreted by Thyroid gland:

Thyroid gland secretes three hormones: 

• Triiodothyronine or T3 
• Tetraiodothyronine or T4 or Thyroxine
• Calcitonin

  A.  T3 and T4 hormones

(i) Secreted by: 

T3 and T4 are secreted by follicular epithelial cells of thyroid.

(ii) Building block of T3 and T4:

Iodine is the building block of T3 and T4 hormones. Both T3 and T4 hormones are iodinated form of amino acid Thyronine.

• Source of iodine:

Iodine is not naturally produced in our body, so we need to take it in external form such as intake of salt along with the food.

The iodine present in food gets absorbed from the bowel into the bloodstream and is transported to the target organ i.e., thyroid gland.

(iii) Amount of T3 and T4 hormones to be released 

(Example of Negative-Feedback Mechanism)

The amount of thyroid hormones to be released into the bloodstream is controlled by the hypothalamus and master gland "Pituitary Gland" through Negative-Feedback mechanism.

Hypothalamus release TSH Releasing Hormone (TRH) which signals Pituitary gland to produce more or less amount of hormones.

a) If the levels of T3 and T4 hormones are low in the blood, then Pituitary gland produces more Thyroid Stimulating Hormone (TSH) which stimulates Thyroid gland to produce more quantities of T3 and H4 hormones.

b) If the levels of T3 and T4 hormones are high in blood, then Pituitary gland produces lesser amount of TSH which stimulates Thyroid gland to produce lesser quantities of T3 and H4 hormones.

(iv) Function:

T3 and T4 hormones controls the basal metabolic rate of the body such as:

• Body temperature,
• Heart rate,
• Breathing rate
• Body weight
• Muscle strength
• Cholesterol levels
• Controls Central Nervous System (CNS) and Peripheral Nervous System (PNS)

(v) Undersecretion of T3 and T4 hormones (Hypothyroidism):

Hypothyroidism is caused when insufficient amount of T3 and T4 hormones are produced in the body.

Hypothyroidism can cause:

a) Simple goitre:

• Symptoms: Enlargement of thyroid gland, visible as swollen neck.
• Causes: Insufficient amount of iodine intake in food

b) Cretinism:

Cretinism affects growth of children.

• Symptoms: Dwarfinism, Mental Retardation
• Causes: Defective development of thyroid gland or atrophy (degeneration) of thyroid gland.

c) Myxoedema:

Myxoedema also known as Gull's disease, occurs in adults.

• Symptoms: Swelling on hands and face, person becomes sluggish
• Causes: Thyroid gland does not function properly

(vi) Oversecretion of T3 and T4 hormones (Hyperthyroidism):

Hyperthyroidism is caused due to oversecretion of T3 and T4 hormones.

 

Hyperthyroidism causes:

a) Exophthalmic goitre

(Exo means "outward", ophthamos means "eyes")

• Symptoms: Protruding eyes, Shortness of breath, rapid heart rate, increased metabolic rate.
• Causes: Oversecretion of T3 and T4 hormones

  B.  CALCITONIN:

Procalcitonin (PCT) is a 116-amino acid peptide which acts as the precursor of Calcitonin. PCT cleaves to form 32 amino acids peptide Calcitonin, which is a non-iodinized hormone.

(i) Secreted by:

C-cells or Parafollicular cells of thyroid gland. 

(ii) Function:

Calcitonin controls calcium and bone metabolism.

(iii) Calcitonin vs Parathormone

Calcitonin decreases the serum calcium levels by decreasing the calcium absorption by:

• osteoclastic cells of the bone
• renal tubules of kidney

Calcitonin secretion is stimulated when serum calcium levels become too high.

Thus, it works antagonistic (opposite) to Parathormone which increases serum calcium levels by increasing calcium resorption by:

• osteoclastic cells of the bone
• renal tubules of kidney

Parathormone secretion is stimulated when serum calcium levels become too low.

Reference:

NCBI