6. THYROID gland

Question 1

where is the THYROID LOCATED

Answer 1

• below Larynx
• anterior to upper part of Trachea
• overlies 2nd, 3rd, 4th Tracheal Cartilage

Question 2

approx weight of Thyroid

Answer 2

25g

Question 3

the LOBES of the THYROID contain…

Answer 3

FOLLICLES
- the functional units of the thyroid gland

Question 4

Interspersed between thyroid FOLLICLES are…

Answer 4

C-CELLS
- secrete CALCITONIN
(calcium homeostasis - decreases)

Question 5

THYROID FOLLICLES consist of a LAYER of…

Answer 5

EPITHELIUM (CUBOIDAL)

Question 6

FOLLICLES have CENTRAL CAVITIES containing…

Answer 6

COLLOID

Question 7

major constituent of COLLOID from CAVITIES of follicles (stores..)

Answer 7

large glycoprotein : THYROGLOBULIN

Question 8

FOLLICLE CELLS are …. DEPENDENT

Answer 8

TSH DEPENDENT

TSH regulates the activity of follicles and the synthesis and release of T3 & T4

(TSH from pituitary)

Question 9

what are essential for the SYNTHESIS of THYROID HORMONES - T3 & T4 (2)

Answer 9

TYROSINE (amino acid)
IODINE

both taken up from blood into tissues

Question 10

TYROSINE (essential amino acid) is SYNTHESISED by…

Answer 10

by the BODY
(in the THYROGLUBULIN)

Question 11

IDOINE is a…

Answer 11

DIETARY ESSENTIAL

Question 12

Other endocrine glands secretes their hormones once produced, the THYROID GLAND…

Answer 12

STORES considerable amount of the thyroid hormones in
the colloid until needed

Question 13

THYROGLOBULIN is PRODUCED by..

Answer 13

FOLLICULAR CELLS

Question 14

how does THYROGLOBULIN get into COLLOID

Answer 14

RELEASED into COLLOID in follicle lumen by EXOCYTOSIS

(produced by follicular cells)

Question 15

how does IODINE get into COLLOID (what happens before)

Answer 15

IDODIDE (I-) UPTAKE from BLOOD into FOLLICULAR CELL

undergoes OXIDATION

transferred to COLLOID

Question 16

how are DI-IODOTYROSINE and MONO-IODOTYROSINE formed in the colloid (DIT & MIT)

Answer 16

TYROSINE RESIDUES are added to IODINE

Question 17

COUPLING PROCESSES between the IODINATED TYROSINE MOLECULES (DIT and MIT) form…

Answer 17

T4 (2 DITs)
T3 (DIT+MIT)

Question 18

SECRETION of T3 and T4 upon stimulation occurs by… (4 steps)

Answer 18

• ENDOCYTOSIS
• FUSION with a LYSOSOME
• UNCOUPLING of T3 & T4
• DIFFUSION OUT OF FOLLICULAR CELL INTO BLOOD and ONTO PERIPHERAL TISSUES

Question 19

5 Steps in biosynthesis, storage
and secretion of thyroid hormones T3, T4

Answer 19

• THYROGLOBULIN produced by FOLLICULAR CELLS and
  released into colloid in follicle lumen by EXOCYTOSIS
  (thyroglobulin produces tyrosine)
• IODIDE (I-) UPTAKE by follicular cell from the
  blood, OXIDATION(I) and transferred to COLLOID
• Attachments of IODINE into TYROSINE RESIDUES
  on thyroglobulin in colloid forming DI- and
  MONO- IODOTYROSINE (DIT and MIT)
• COUPLING PROCESSES between the iodinated
  tyrosine molecules to form T4 (2DIT) and T3
  (DIT + MIT)
• SECRETION (upon stimulation) of T4 and T3
  occurs by ENDOCYTOSIS,
  fusion with a LYSOSOME,
  UNCOUPLING of T4 and T3
  and DIFFUSION out of the follicular cell into the blood and onto peripheral tissues

Question 20

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

Answer 20

T4

however, MAJORITY of T4 secreted is subsequently CONVERTED TO T3

Question 21

T3 is FORMED from….

Answer 21

MONODEIODINATION OF T4
in thyroid and peripheral tissues

Question 22

which thyroid hormone is MORE POTENT in its biological form and is the MAJOR HORMONE that interacts with target cells

Answer 22

T3
- 4X more potent than T4

Question 23

CONVERSION OF T4 to T3 primarily occurs where? (2)

(MONODEIODINATION)

Answer 23

LIVER
KIDNEY

Question 24

Regulation of thyroid hormone secretion
starting from Hypothalamus…

Answer 24

HYPOTHALAMUS: secretes TRH (THYROTOPIN RELEASING HORMONE)

ANTERIOR PITUITARY: TSH (THYROID STIMULATING HORMONE)

THYROID GLAND: T3, T4

• to target cells

Question 25

TSH RECEPTORS are members of which receptor family

Answer 25

G-PROTEIN COUPLED RECEPTORS

• second messenger cAMP

Question 26

Thyroid hormones are involved in regulation of… (2)

Answer 26

GROWTH (mainly in growing children)
& the NERVOUS SYSTEM

Question 27

Thyroid Hormone STIMULATES which hormone secretion

Answer 27

GROWTH HORMONE SECRETION
- and promotes GH EFFECTS

Question 28

Thyroid Hormone is important in promoting GROWTH and DEVELOPMENT of the…

Answer 28

BRAIN
- during FETAL and POSTNATAL LIFE

Question 29

DEFICIENCY/LACK of thyroid hormone can result in (2)

Answer 29

GROWTH RETARDATION
- but can be reversed by thyroid replacement therapy
( excess thyroid hormone does NOT produce excessive growth )

MENTAL RETARDATION if therapy is not administered days or weeks after birth

Question 30

PHYSIOLOGICAL effects of thyroid hormones

Answer 30

INCREASE METABOLIC activities

INCREASE OXYGEN CONSUMPTION to most metabolically active tissues

INCREASE BMR (basal metabolic rate) significantly - by 60-100%

increased metabolism means INCREASED HEAT PRODUCTION
- thyroid hormone is CALORIGENIC (HEAT PRODUCING)

Modulates rates of many specific reactions involved in fuel
metabolism

Question 31

SYMPATHOMIMETIC effect of thyroid hormones

Sympathomimetic: any action SIMILAR to one produced by the SYMPATHETIC NERVOUS SYSTEM

Answer 31

*INCREASED target cell RESPONSIVENESS to CATECHOLAMINES (SNS and adrenal glands)

*Thyroid hormones are permissive →INCREASED PRODUCTION of SPECIFIC CATECHOLAMINE target cell RECEPTORS

Question 32

effects of thyroid hormones in CARDIOVASCULAR SYSTEM

Answer 32

*↑ INCREASE the HEART’S RESPONSIVENESS to circulating CATECHOLAMINES

*↑ INCREASED HEART RATE and FORCE OF CONTRACTION → ↑INCREASED CARDIAC OUTPUT

*In response to the HEAT load → peripheral VASODILATION to ELIMINATE generation of extra HEAT

Question 33

METABOLIC EFEFCTS of thyroid hormones (7)

Answer 33

*Lipid metabolism
*Carbohydrate metabolism
*GROWTH
*Development (fetal and neonatal brain)
*CARDIOVASCULAR system
*Central nervous system CNS
* REPRODUCTIVE system

Question 34

T3 & T4 DEFICIENCY condition

Answer 34

HYPOTHYROIDISM

Question 35

SYMPTOMS of HYPOTHYROIDISM

Answer 35

*Fatigue, Lethargy
*Weakness
*Cold intolerance (due to low BMR)
*Mental slowness
*Depression
*Dry skin
*Constipation
*Muscle cramps
*Irregular menses
*Infertility
*Mild weight gain
*Fluid retention
*Hoarseness

Question 36

SIGNSof Hypothyroidism

Answer 36

*Goitre (primary hypothyroidism only) (visible swelling of thyroid gland)
*Bradycardia (slow heart rate)
*Non-pitting edema (swelling due to excess fluid)
*Dry skin
*Delayed relaxation
*Hypertension (high blood pressure)
*Slow speech
*Slow movements
*Voice hoarseness

Question 37

CAUSES of Hypothyroidism

Answer 37

• AUTOIMMUNE (Hashimoto’s) thyroiditis
• ATROPHIC (common in elderly)
  ATROPHY of thyroid - decreasing in size & wearing away
• related to AGE
• Iodine 131 treatment
• pituitary - secondary to PITUITARY disease

Question 38

How to evaluate if suspect Hypothyroidism

Answer 38

look at TSH CONCENTRATIONS

• INCREASED TSH is the most sensitive test (PRIMARY hypothyroidism)
  (lack of T4,T4 means loss of negative feedback so increased TSH)
• need to know if there is underlying pituitary disease (defective TSH secretion) - SECONDARY hypothyroidism
• if suspect primary hypothyroidism, measure ANTIBODIES
  Antibodies in hypothyroidism are ASSOCIATIONS not causal
  (very high titres suggest Hashimoto’s thyroiditis )

antibodies: Anti-thyroid peroxidase (TPO) and anti-thyroglobulin (Tg ) antibodies (Hashimoto’s)

Question 39

Hypothyroidism Therapy - replace T4 / levothyroxine

Goals:

Answer 39

• Alleviate symptoms
• Titrate T4 intake to normalize TSH (primary
  hypothyroidism) or free T4 (secondary and tertiary
  hypothyroidism

Question 40

What CONVERTS T4 to T3
(Liver and kidney)

Answer 40

TYPE 1 5’-DEOIDINASE

Question 41

Follicular cells are … dependent?

Answer 41

TSH DEPENDENT