PHYSIOLOGY OF THE THYROID GLAND

BY: SAFANA SADIQ

PHYSIOLOGY OF THE

THYROID GLAND

CONTENT

Anatomy of the thyroid gland

Synthesis and secretion of thyroid hormones

Regulation of thyroid hormone secretion

Physiological functions of the thyroid hormone

Diseases of the thyroid

The endocrine system consists of ductless glands that produces hormones which pass into the blood stream for general circulation to excite or inhibit the activity of other organs or tissues.

The thyroid gland is one of the largest endocrine glands.

It secretes 2 major hormones:

Thyroxine – T4

Triiodothyronine – T3

ANATOMY

The thyroid gland is located immediately below the larynx and anterior to the upper part of the trachea. It weighs about 25g.

It consists of 2 lateral lobes connected by a narrow band of thyroid tissue called the isthmus.

The isthmus usually overlies the region from the 2nd to 4th tracheal cartilage.

HISTOLOGY

The lobes of the thyroid contain many hollow, spherical structure called follicles, which are the functional units of the thyroid gland.

Interspersed between the follicles are C cells, which secrete calcitonin.

Each follicle is filled with a thick sticky substance called colloid.

The major constituent of colloid is a large glycoprotein called thyroglobulin.

Unlike other endocrine glands, which secretes their hormones once they are produced, the thyroid gland stores considerable amount of the thyroid hormones in the colloid until they are needed by the body.

SYNTHESIS, STORAGE & SECRETION OF THYROID HORMONES

Tyrosine and iodine are essential for synthesis of thyroid hormones.

Both are taken up by the blood

Tyrosine is synthesised by the body (in the thyroglobulin).

Iodine is a dietary essential.

Hormone synthesis occurs on the thyroglobulin.

SYNTHESIS, STORAGE & SECRETION

Following steps are involved synthesis, storage and secretion:1. Thyroglobulin production by follicular cell and released into colloid

by exocytosis

2. Iodine uptake by follicular cell from the blood and transferred to colloid

3. Attachments of iodine to tyrosine on thyroglobulin in colloid

4. Coupling processes between the iodinated tyrosine molecules to form T4 and T3

5. Secretion (upon stimulation) of T4 and T3 occurs by endocytosis a piece of colloid, uncouplingof T4 and T3 and diffusion out of the follicular cell into the blood

SYNTHESIS, STORAGE & SECRETION cont..

SYNTHESIS, STORAGE & SECRETION cont..

Approximately, 90% of the hormones released from the thyroid gland initially appear in the form of T4.

However, a majority of the T4 that is secreted from the thyroid gland is subsequently converted to T3.

T3 is 4times more potent in its biologic form than T4 and is the major hormone that interacts with the target cells.

SYNTHESIS, STORAGE & SECRETION cont..

Both the hormones are highly lipophilic and once in the blood, immediately bind to proteins: Thyroid hormone specific protein- thyroxine binding globulin

(70-80%)

Other non-specific proteins

Less than 0.1% of T4 and 1% of T3 are in unbound form.

These free T4 and T3 compounds are biologically active, and it is these components that produce the effects of the thyroid hormones on peripheral tissues and on the pituitary feedback mechanism.

REGULATION OF THRYOID HORMONE SECRETION

REGULATION OF THYROID HORMONE SECRETION

PHYSIOLOGICAL EFFECTS OF THYROID HORMONES

Metabolic rate and heat production: ↑ metabolic activities

↑ O₂ consumption to most metabolically active tissues

BMR can ↑ by 60 – 100%

Since ↑ metabolism results in ↑ heat production → thyroid

hormone effects is calorigenic

Intermediary metabolism:

Modulates rates of many specific reactions involved in fuel metabolism

PHYSIOLOGICAL EFFECTS OF THYROID HORMONES

Sympathomimetic effect Sympathomimetic: any action similar to one produced by the

sympathetic nervous system Thyroid hormone ↑ target cell responsiveness to catecholamines

(SNS and adrenal) Thyroid hormones are permissive → ↑ production of specific

catecholamine target cell receptors

The cardiovascular system: ↑ the heart’s responsiveness to circulating catecholamines

(permissive action above) ↑ heart rate and force of contraction → ↑ CO In response to the heat load → peripheral vasodilation to eliminate

generation of extra heat

PHYSIOLOGICAL EFFECTS OF THYROID HORMONES

Growth and the nervous system

Effect on growth is manifested mainly in growing children Thyroid hormone stimulates GH secretion and promotes GH effects

Lack of thyroid hormone → growth retardation but can be reversed

by thyroid replacement therapy

Unlike excess GH, excess thyroid hormone does not produce excessive growth

Important in promoting growth and development of the brain during fetal and postnatal life Thyroid hormone deficiency → mental retardation if therapy is not

administered days or weeks after birth

PHYSIOLOGICAL EFFECTS OF THYROID HORMONES

DISEASES OF THE THYROID GLAND

Thyroid hormones can be either deficient or excessive – hypothyroidism or hyperthyroidism.

UNDERACTIVITY OF THYROID

If the thyroid show atrophy or destruction of its secretory cells or is inadequately stimulated, the syndrome of hypothyroidism develops because of lack of thyrotrophin releasing hormone from the hypothalamus or TSH from the anterior pituitary gland.

HYPOTHYROIDISM

Symptoms:

↓ overall metabolic activity, including ↓ BMR

Poor tolerance of cold → lack of calorigenic effect

Tendency to gain weight → not burning fuel at a normal rate

Gut movements sluggish →constipation

Easily fatigue → lower energy production

Heart and respiratory rates and BP reduced.

Mental effects including ↓ alertness, slow speech and poor memory.

HYPOTHYROIDISM

Symptoms:

Hair- brittle, sparse, dry.

Slow, husky voice.

Myxedema → Retention of water presumably by

water retaining carbohydrates as a result of altered metabolism.

Generically, treatment consist of replacement therapy:

Thyroid hormone

Iodine

HYPOTHYROIDISM

HYPERTHYROIDISM

Most common cause –Grave’s disease

Autoimmune disease where thyroid-stimulating immunoglobulin (TSI) is produced erroneously

Is not subject to the negative feedback mechanism

HYPERTHYROIDISM

Symptoms:

BMR is ↑, more heat is produced → ↑body temperature

Skin is hot and flushed

Appetite increase but weight falls

Movements of digestive tracts are ↑ → diarrhea

Heart and respiratory rates ↑

↑ BP

A fine muscular tremor and nervousness are marked.

Person becomes excitable, irritable and apprehensive.

Symptoms:

Exophthalmos →complex water-retaining carbohydrate are deposited behind the eye → fluid

retention pushes eyeballs forward (prominent feature of Grave’s disease only).

Treatment for hyperthyroidism is anti-thyroid drugs.

THYROID DISEASE

Goiter

May occur in hypothyroidism and hyperthyroidism but does not need to be present in either conditions

CASE STUDY

A 39-year-old woman presented with a 3month history of increased sweating and palpitations with weight loss of 7kg. She had family history of thyroid disease, her maternal aunt had suffered from thyroid disease.

On examination, she was a nervous, agitated woman with an obvious, diffuse, non-tender, smooth enlargement of her thyroid, over which a bruit could be heard. She had a fine tremor of her fingers and a resting pulse rate of 150/minute. She had no evidence of exophthalmos.

CASE STUDY cont…

On investigation, she had a raised serum T3 of 4.8nmol/l (NR 0.8-2.4) and a T4 of 48nmol/l (NR 9-23). Measurement of her thyroid-stimulating hormone showed that this was low normal, 0.4mU/l (NR 0.4-5_mU/l). The biochemical findings pointed to primary thyroid disease.

A diagnosis of autoimmune thyrotoxicosis (Graves' disease) was made. She was treated with an antithyroid drug, carbimazole, to control her thyrotoxicosis, and surgery was not required.

REFERENCES:

Callander R, Mackenna B: Illustrated Physiology, 6th Edition

Ganong, WF: ‘Review of Medical Physiology’, 23rd Edition

Guyton, AC and Hall JE: ‘Textbook of Medical Physiology’, 12th Edition

Tortora, GJ and Derrickson, B: ‘Principles of anatomy and physiology, 13th Edition

THE END