Thyroid Hormone

THYROID HORMONE
powerpoint presentation by: D. C. MIKULECKY
PROFESSOR OF PHYSIOLOGY

THE THYROID GLAND

* OVER TRACHEA
* TWO LARGE LATERAL LOBES CONNECTED BY AN ISTHMUS
* 15 to 20 g
* FUNCTIONAL UNIT IS THE FOLLICLE: EPITHELIAL CELLS AROUND A HOLLOW VESSICLE FILLED WITH THYROGLOBULIN

THE THYROID HORMONES

* THYROGLOBULIN: STORAGE FORM BINDS HORMONES
* TETRAIODOTHYRONINE
* TRIIODOTHYRONINE
* IODINE REQUIRED FROM DIETARY INTAKE

THYROID HORMONES
THYROID HORMONE SYNTHESIS

* DEPENDENT ON IODINE (IODINE PUMP CONCENTRATES IODINE IN CELLS)
* DEPENDENT ON TYROSINE
* PARTIALLY SYNTHESIZED (THYROGLOBULIN) EXTRACELLULARLY AT LUMINAL SURFACE OF FOLLICULAR CELLS AND STORED IN FOLLICULAR LUMEN

EFFECTS OF IODINE

* DURING IODINE DEFICIENCY, HORMONE SYNTHESIS IS IMPAIRED
* EXCESS IODINE ALSO INHIBITS SYSNTHESIS

THYROID HORMONE SECRETION

* WITH TSH STIMULATION, ENDOCYTOSIS BRINGS THE THYROGLOBIN BACK INTO FOLLICULAR CELLS
* THYROGLOBULIN IS DEGRADED TO T3 AND T4

THYROID HORMONE’S EFFECTS

* METABOLIC RATE: INCREASED BMR
* CALOROGENIC: INCREASED HEAT PRODUCTION(OXIDATIVE METABOLISM)
* SYMPATHOMIMETIC: FLIGHT OR FIGHT
* CARDIOVASCULAR:INCREASES RESPONSIVENESS OF HEART
* GROWTH: ESSENTIAL FOR NORMAL GROWTH OF SKELETAL SYSTEM (PERMISSIVE OR SYNERGYSTIC WITH GH, INSULIN-LIKE GROWTH FACTOR), CNS, ANS
* NERVOUS SYSTEM:DEVELOPMENT AND ADULT ACTIVITY

METABOLIC EFFECTS OF THYROID HORMONE

* CALOROGENIC EFFECT: INFLUENCES TOLERENCE TO COLD, AVAILABILITY OF ATP
* CARBOHYDRATE METABOLISM: INCREASED GLUCOSE ABSORPTION FROM GUT,GLCOGENOLYSIS, GLUCONEOGENESIS, GLUCOSE OXIDATION.
* LIPID METABOLISM: LIPOGENESIS IN ADIPOCYTES, IN COORDIMNATION WITH BLOOD GLUCOSE LEVELS

CONTROL OF FUEL METABOLISM

* GLYCOGENESIS
* GLYCOGENOLYSIS
* GLUCONEOGENESIS
* PROTEIN SYNTHESIS
* PROTEIN DEGRADATION
* FAT SYNTHESIS
* FAT BREAKDOWN


GLYCOGENESIS

* GLYCOGEN IS A BRANCHED POLYMER OF GLUCOSE STORED IN THE LIVER AND MUSCLE CELLS
* SYNTHESIS IS BY SEPARATE PATHWAY FROM BREAKDOWN
* HIGHLY REGULATED BY INSULIN

GLYCOGENOLYSIS

* BREAKDOWN OF GLYCOGEN STORES INTO GLUCOSE
* REGULATES BLOOD GLUCOSE BETWEEN MEALS
* HOMONALLY CONTROLLED (GLUCAGON, EPINEHRINE, NOREPINEPHRINE AND CLUCOCORTICOIDS) AMPLIFIED BY THYROID HORMONE

GLUCONEOGENESIS

* PRECURSORS ARE 3 AND 4 CARBON COMPOUNDS
* VIA FRUCTOSE PHOSPHATE
* GLUCAGON CONTROLLED AIDED BY THYROID HORMONE
* MAIN PRECURSOR ALANINE AND OTHER AA


PROTEIN DEGRADATION

* USUALLY BALANCED BY SYNTHESIS
* NO ENERGY STORES IN FORM OF PROTEIN
* CAN BE ENHANCED BY GLUCAGON AND THYROID HORMONES LEADING TO GLUCONEOGENESIS

THYROID HORMONE EFFECTS ON NITROGEN METABOLISM

* ENHANCES BOTH SYNTHESIS AND DEGRADATION OF PROTEINS
* EXCESS HORMONE PROMOTES DEGREDATION

FAT SYNTHESIS

* GLUCOSE - FATTY ACID CYCLE
* FATTY ACIDS PRODUCED CONSTANTLY IN ADIPOSE TISSUE.
* BECOME FFA OR BECOME TRIGLYCERIDES DEPENDING ON -GLYCEROL PHOSPHATE FROM GLUCOSE OXIDATION
* NEED OPTIMAL AMOUNTS OF THYROID HORMONE

GLUCOSE - FATTY ACID CYCLE

THYROID AND TEMPERATURE REGULATION

* T3 IS THE DOMINANT FORM INVOLVED
* EXPOSURE TO COLD CAUSES T4 CONVERSION TO T3 .
* PROMOTES CALOROGENIC EFFECT (LONG TERM COLD ADAPTATION)
* SHORT TERM EFFECTS DUE TO SYMPATHETIC MIMETIC EFFECTS AND THE SHIVERING RESPONSE OF MUSCLES

REGULATION OF THYROID SECRETION
EFFECTS OF TSH
TSH MODE OF ACTION
ABNORMALITIES OF THYROID FUNCTION
THYROID HORMONE.ppt