Endocrine Physiology (PHS 327) - PowerPoint Presentation

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Endocrine Physiology (PHS 327)

Physiology Program

College of Health Sciences

Bowen University, Iwo, Nigeria

Dr Michael Olugbenga S

Physiology of the

Thyroid Gland

Outline

Thyroid glandular cells

Hormones of the thyroid gland

Thyroid hormone synthesis

Functions of

thyroxine

Thyroxine

regulation/control

Clinical correlates

Human thyroid

Thyroid cell

Source: Ganong 24

th

edition

Anatomy of the thyroid gland

Introduction

The thyroid gland, located immediately below the larynx on each side of and anterior to the

trachea.

It is one

of the largest of the endocrine

glands.

normally

weighing 15 to 20

g in adults.

The thyroid secretes three hormones; thyroxine (T4),

triiodothyronine (T3), and calcitonin.

T3 & T4 :

secreted by the follicular cells

Calcitonin

:

secreted by the parafollicular cells

T3 & T4 profoundly

increase the metabolic rate of the body.

Calcitonin

is an

important hormone for calcium metabolism

Ganong 24

th

edition

Chemical structure of T3 & T4

Ganong 24

th

edition

Thyroid hormone synthesis

Iodine is an essential raw material for

thyoid

hormone synthesis.

The minimum iodine intake for normal thyroid function in an adult is 150µg

Steps involved in thyroid hormone synthesis:

Active uptake of inorganic iodide (I

-

).- iodide trapping

Oxidation of I

-

to organic iodine I

2

. by

peroxidase

Iodination of tyrosine to produce mono &

diiodotyrosine

.

Oxidative coupling to form T3 & T4.

Cellular mechanism of thyroid hormone formation

Guyton and Hall, 2011

Transport

Bound

Free

Plasma level

99.8%

0.2%

0.15 mcg%

T3

99.98%

0.02%

8 mcg%

T4

The transport proteins are:

Thyroid binding globulin (TBG) (

α

-globulin) (50%)

Thyroxine binding prealbumin (TBPA). (40%)

Plasma albumin. (10%).

T 4 and T 3 are

deiodinated

in the liver, the kidneys, and

many other

tissues

.

About 93% of

the thyroid hormone released from the thyroid gland is normally

thyroxine and only 7 percent is

triiodothyronine.

However, during the ensuing few days, about one half of the thyroxine

is slowly

deiodinated

to form additional

triiodothyronine

.

Therefore

, the hormone finally delivered to and

used

by the tissues is mainly

triiodothyronine

, a total of about 35 micrograms of

triiodothyronine

per day

.

Regulation of Thyroid Secretion

Thyroid

function is

controlled by

variations in the

circulating level

of pituitary

TSH

TSH secretion

is increased by

the hypothalamic hormone TRH and inhibited in a negative feedback fashion by circulating

free T

4 and T 3

.

TSH secretion is also inhibited by stress, and in

experimental animals

it is increased by cold and decreased by warmth

.

The

negative feedback

effect

of thyroid

hormones on

TSH secretion is exerted in part at the

hypothalamic level,

but it is also due in large part to an action on the pituitary,Since

T 4 and T 3 block the increase in TSH secretion produced by TRH.

Infusion of either T 4 or T 3 reduces the circulating level of TSH, which declines measurably within

1h.The day-to-day maintenance of thyroid secretion depends on the feedback interplay of thyroid hormones with TSH and TRH

Regulation of thyroid hormone

synthesis

Adapted from Ganong 24

th

edition

Hypothalamus

TRH

(-)

(-)

(-)

Mechanism of Action

Guyton and Hall, 2011

Functions of Thyroid hormone

Thyroid Hormones Increase Cellular Metabolic

Activity

Thyroid Hormones Increase the Number and Activity of

Mitochondria

Th

yroid

hormones are essential for normal growth and skeletal maturationThyroid hormones lower circulating cholesterol levels

Essential for normal function of the skeletal muscle.Stimulates erythropoiesis

Stimulate secretion of digestive juices (including gastric

HCl

).

increased appetite.

- Stimulate intestinal motility & absorption.

Increased rate & depth of respiration

Increased heart rate

Increased strength of myocardial contractility

Increased cardiac output.

Arterial blood pressure

Enhance action of insulin.

Stimulate

glycogenesis

.

Stimulate glucose utilization

Normal levels of thyroid hormones stimulate protein synthesis & growth (anabolic).

- Excess thyroid hormone causes protein catabolism

Essential for normal

menestrual

cycle & spermatogenesis (fertility)

- Stimulates milk secretion during lactation.

- Growth, development & function of CNS during fetal life & 1st few years after birth.

- Myelination of nerve fibers & development of synapses.

Disorders of the Thyroid Gland

Hypofunction (hypothyroidism)

Cretinism

Myxoedema

Hyperfunction (hyperthyroidism)