6. THYROID gland Flashcards
where is the THYROID LOCATED
- below Larynx
- anterior to upper part of Trachea
- overlies 2nd, 3rd, 4th Tracheal Cartilage
approx weight of Thyroid
25g
the LOBES of the THYROID contain…
FOLLICLES
- the functional units of the thyroid gland
Interspersed between thyroid FOLLICLES are…
C-CELLS
- secrete CALCITONIN
(calcium homeostasis - decreases)
THYROID FOLLICLES consist of a LAYER of…
EPITHELIUM (CUBOIDAL)
FOLLICLES have CENTRAL CAVITIES containing…
COLLOID
major constituent of COLLOID from CAVITIES of follicles (stores..)
large glycoprotein : THYROGLOBULIN
FOLLICLE CELLS are …. DEPENDENT
TSH DEPENDENT
TSH regulates the activity of follicles and the synthesis and release of T3 & T4
(TSH from pituitary)
what are essential for the SYNTHESIS of THYROID HORMONES - T3 & T4 (2)
TYROSINE (amino acid)
IODINE
both taken up from blood into tissues
TYROSINE (essential amino acid) is SYNTHESISED by…
by the BODY
(in the THYROGLUBULIN)
IDOINE is a…
DIETARY ESSENTIAL
Other endocrine glands secretes their hormones once produced, the THYROID GLAND…
STORES considerable amount of the thyroid hormones in
the colloid until needed
THYROGLOBULIN is PRODUCED by..
FOLLICULAR CELLS
how does THYROGLOBULIN get into COLLOID
RELEASED into COLLOID in follicle lumen by EXOCYTOSIS
(produced by follicular cells)
how does IODINE get into COLLOID (what happens before)
IDODIDE (I-) UPTAKE from BLOOD into FOLLICULAR CELL
undergoes OXIDATION
transferred to COLLOID
how are DI-IODOTYROSINE and MONO-IODOTYROSINE formed in the colloid (DIT & MIT)
TYROSINE RESIDUES are added to IODINE
COUPLING PROCESSES between the IODINATED TYROSINE MOLECULES (DIT and MIT) form…
T4 (2 DITs)
T3 (DIT+MIT)
SECRETION of T3 and T4 upon stimulation occurs by… (4 steps)
- ENDOCYTOSIS
- FUSION with a LYSOSOME
- UNCOUPLING of T3 & T4
- DIFFUSION OUT OF FOLLICULAR CELL INTO BLOOD and ONTO PERIPHERAL TISSUES
5 Steps in biosynthesis, storage
and secretion of thyroid hormones T3, T4
- 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
Approximately, 90% of the hormones released from the
thyroid gland initially appear in the form of…
T4
however, MAJORITY of T4 secreted is subsequently CONVERTED TO T3
T3 is FORMED from….
MONODEIODINATION OF T4
in thyroid and peripheral tissues
which thyroid hormone is MORE POTENT in its biological form and is the MAJOR HORMONE that interacts with target cells
T3
- 4X more potent than T4
CONVERSION OF T4 to T3 primarily occurs where? (2)
(MONODEIODINATION)
LIVER
KIDNEY
Regulation of thyroid hormone secretion
starting from Hypothalamus…
HYPOTHALAMUS: secretes TRH (THYROTOPIN RELEASING HORMONE)
ANTERIOR PITUITARY: TSH (THYROID STIMULATING HORMONE)
THYROID GLAND: T3, T4
- to target cells
TSH RECEPTORS are members of which receptor family
G-PROTEIN COUPLED RECEPTORS
- second messenger cAMP
Thyroid hormones are involved in regulation of… (2)
GROWTH (mainly in growing children)
& the NERVOUS SYSTEM
Thyroid Hormone STIMULATES which hormone secretion
GROWTH HORMONE SECRETION
- and promotes GH EFFECTS
Thyroid Hormone is important in promoting GROWTH and DEVELOPMENT of the…
BRAIN
- during FETAL and POSTNATAL LIFE
DEFICIENCY/LACK of thyroid hormone can result in (2)
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
PHYSIOLOGICAL effects of thyroid hormones
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
SYMPATHOMIMETIC effect of thyroid hormones
Sympathomimetic: any action SIMILAR to one produced by the SYMPATHETIC NERVOUS SYSTEM
*INCREASED target cell RESPONSIVENESS to CATECHOLAMINES (SNS and adrenal glands)
*Thyroid hormones are permissive →INCREASED PRODUCTION of SPECIFIC CATECHOLAMINE target cell RECEPTORS
effects of thyroid hormones in CARDIOVASCULAR SYSTEM
*↑ 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
METABOLIC EFEFCTS of thyroid hormones (7)
*Lipid metabolism
*Carbohydrate metabolism
*GROWTH
*Development (fetal and neonatal brain)
*CARDIOVASCULAR system
*Central nervous system CNS
* REPRODUCTIVE system
T3 & T4 DEFICIENCY condition
HYPOTHYROIDISM
SYMPTOMS of HYPOTHYROIDISM
*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
SIGNSof Hypothyroidism
*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
CAUSES of Hypothyroidism
- 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
How to evaluate if suspect Hypothyroidism
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)
Hypothyroidism Therapy - replace T4 / levothyroxine
Goals:
- Alleviate symptoms
- Titrate T4 intake to normalize TSH (primary
hypothyroidism) or free T4 (secondary and tertiary
hypothyroidism
What CONVERTS T4 to T3
(Liver and kidney)
TYPE 1 5’-DEOIDINASE
Follicular cells are … dependent?
TSH DEPENDENT
