Thyroid Gland 甲状腺 Huiping Wang ( 王会平 ), PhD Department of Physiology Rm C541, Block C, Research Building, School of Medicine Tel: 88208292
Outline Thyroid Hormones Types Biosynthesis Storage and Release Transport Physiological Functions Mechanisms of TH action Regulation of thyroid hormone secretion Thyroid disorders
The Thyroid Gland Located at anterior part of neck on either side of trachea The thyroid gland straddles the trachea, just below the larynx, in the neck. One of the largest endocrine glands Comprised of 2 lobes; 20 g wt Rich in blood supply Thyroid gland becomes functional early in fetal life as it plays a role in the development of brain cells (forming nerve terminals/ synapse/ dendrites/ myelin)
The Thyroid Gland Thyroid follicular cell T 4 and T 3 increase metabolic rate 4 3 Parafollicular cell (C cell) Calcitonin regulates calcium metabolism
Thyroid Gland Follicle The thyroid gland is composed of many follicles Each follicle contains colloid (thyroglobulin, TG +hormones) Colloid is absorbed back into the follicle epithelial cells upon stimulation Follicular cells take part in almost all stages of TH synthesis and secretion
Types of Thyroid Hormones 90% of metabolically active hormones secreted by thyroid is thyroxine (T 4 ), 10% is triiodothyronine (T 3 ), and less than 1% is rt3 Iodide is required (1mg/week) for thyroid hormone synthesis
Thyroid Hormones Thyroxine (T 4 ) Major hormone released from thyroid follicles Contains 4 iodine atoms More abundant than T 3
Thyroid Hormones Triiodothyronine (T 3 ) Generally formed from T 4 by cleaving an iodine molecule (deiodinase in target cells) 4timesmore more potent than T 4 Contains 3 iodine atoms
Biosynthesis of Thyroid Hormones Materials Iodine (in the form of iodide-i - ) Iodide (1mg/week) is required for TH synthesis Iodized table salt (1 sodium iodide/100,000 NaCl) Tyrosine Thyroglobulin (TG) >70 tyrosines
Biosynthesis of Thyroid Hormones Iodine/iodide trapping Active transport Maintains thyroid/plasma ratio of 30:1 Oxidation of iodide TPO 2I - + H 2 O 2 I 0 (iodine free radical) Iodination of tyrosine TPO MIT and DIT Formation of T3, T4 by coupling TPO MIT + DIT T 3 DIT + DIT T 4 MIT-monoiodotyrosine DIT-diiodotyrosine TPO-thyroid peroxidase
Storage and Release of Thyroid Hormones Storage In the follicles In the form of TG (30 T4) For 2 ~3 months Release Stimulated by TSH Concentration in plasma T 3 : 12 1.2 ~3.4 34nmol/L T 4 : 85 ~142 nmol/l
Transport of T 4 and T 3 to the Tissues Binding form:99% Most T 4 and T 3 bound to plasma proteins synthesized by liver: Thyroxine-binding globulin (TBG): mainly Thyroxine-binding prealbulmin (TBPA) albulmin l Slowly released to the tissue cells, slower for T 4 due to its higher affinity Slow onset and long duration of action Free form:1% ( T 3 )
Physiological Functions of TH TH exerts widespread and diverse effects throughout the body. Effect on Growth Metabolism Nervous System Cardiovascular System GI Muscles, sexual function
Effects of TH on Growth Is essential for growth in children Promote bone formation and maturation Promote growth and development of brain (fetus and neonate) Have synergistic effect with GH, IGF-1, insulin and other growth factors
Effects of TH on Growth Cretinism: -deficiency of thyroid hormone during the period of fetal and early neonatal development -short, stocky stature & mental retardation -reversed with iodine and TH administration shortly after birth
Metabolic Effects of TH Increase cellular metabolic activity Calorigenic action BMR number & activity of mitochondria ATP Na +, K + -ATPase heat production O 2 consumption & BMR; CO 2 production ventilation
Metabolic Effects of TH Carbohydrate metabolism glucose oxidation, effect of E, glucogon, cortisol and GH gluconeogenesis and glycogenolysis Too much blood glucose (Diabetes) Fat metabolism lipolysis (Triglyceride FFA + glycerol) Oxidation of free fatty acid serum cholesterol(excretion hl l( into GI) ) Protein metabolism Protein synthesis (normal) Protein catabolism (hyperthyroidism) -hyperthyroidism: y weight loss - hypothyroidism:myxedema (glycosaminoglycans trapped with water accumulate in the interstitial space) hyperthyroidism hypothyroidism -myxedema
Effects of TH on Nervous System Important t for maturation ti of CNS in perinatal period Enhance wakefulness, alertness, responsiveness to various stimuli, auditory sense, awareness of hunger, memory and learning capacity Normal emotional tone also depend on proper thyroid hormone Increase the speed and amplitude of peripheral nerve reflexes Hyperthyroidism: hyperexcitability, insomnia, loss of concentration Hypothyroidism: mental retardation, sleepiness
A baby suffering from congenital hypothyroidism
Effects of TH on Cardiovascular System Increase blood flow and cardiac output metabolism utilization of O 2 & metabolic end products from tissue vasodilatation cardiac output ensures sufficient O 2 delivery to the tissues Increase heart rate (easy to detect!) increase adrenergic activity( response to adrenaline/noradrenaline) increase enzymatic activity Affect heart strength th slightly increase of thyroid hormone increases heart strength marked increase of thyroid hormone decreases heart strength stroke volume + peripheral resistance pulse pressure
Effects of TH on GI Increase appetite and food intake Increase secretion of digestive juices Increase mobility of GI Diarrhea often results from hyperthyroidism
Effects of TH On muscles hyperthyroidism muscle weakening (catabolic effect) fine muscle tremor is a characteristic sign for hyperthyroidism hypothyroidism muscles sluggish On other endocrine glands secretion of insulin and cortisol On sexual function loss of libido impotency abnormal menstruation
Overall Effects of Thyroid Hormones
Mechanisms of TH action T 3 and T 4 act by binding to nuclear receptors which are expressed in most tissues T 3 has 10 times the affinity for thyroid receptor as T 4 Transcription of large numbers of genes Synthesis y of great numbers of proteins
Regulation of thyroid hormone secretion Hypothalmico-pituitary-thyroid axis TRH TSH T 3 /T 4 Negative feed-back of T 3 /T 4 Environmental factors cold, stress Wolff-Chaikoff Effect autoregulation by I
Hypothalmico-pituitary-thyroid pituitary thyroid axis Hypothalamus TRH Tripeptide TSH secretion Cold: TRH release
Hypothalmico-pituitary-thyroid pituitary thyroid axis Pituitary TSH Glycoprotein T 3, T 4 synthesis and release Size of the thyroid cells Hyperthyroidism thyroid stimulating immunoglobulin (TSI) Anti-TSH receptor antibody Stimulate TSH receptor
Hypothalmico-pituitary-thyroid pituitary thyroid axis Thyroid Feedback effect of thyroid hormone T 4 is converted into T 3 in pituitary and T 3 acts as the final effector to turn off TSH
Iodine-Deficient Goiter TSH is a trophic hormone, it stimulates not only T 3 /T 4 secretion but also protein synthesis in follicular cells. Therefore, size
Wolff-Charkoff effect autoregulation by I low I intake stimulates synthesis of iodide trap high I intake (>2 mg/day) inhibits trap function and synthesis
Summary Types of Thyroid Hormones thyroxine (T4) triiodothyronine (T3) Biosynthesis of Thyroid Hormones Iodine/iodide trapping Oxidation of iodide Iodination of tyrosine Formation of T3, T4 by coupling Storage and Release of Thyroid Hormones Transport of T4 and T3
Summary Physiological Functions of TH Effect on Growth, Metabolism, Nervous System, Cardiovascular System, GI Mechanisms of TH action Regulation of thyroid hormone secretion Hypothalmico-pituitary-thyroid axis Wolff-Chaikoff Effect Thyroid disorders Hyperthyroidism y Hypothyroidism
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