LECTURE 14 - thyroid gland

Question 1

What is the thyroid gland?

Answer 1

• endocrine gland
• secretes hormones directly into bloodstream
• 2 main thyroid hormones
  1. T3 = triiodothyronine
  2. T4 = thyroxine
• -> essential for development, regulation of growth and control of basic metabolic rate

Question 2

Describe the thyroid gland anatomy

Answer 2

• located in neck
• butterfly shaped gland
• 2 lobes united by a narrow isthmus
• brownish-red, highly vascular
• thin fibrous capsule containing follicles and connective tissue

Question 3

What are the thyroid follicular epithelial cells?

Answer 3

• majority of thyroid cells
• line the spherical follicles and surround the follicular lumen filled with colloid (thyroid hormone store)
• controlled by thyroid-stimulating hormone (TSH)
• responsible for production of T3 and T4

Question 4

What are parafollicular cells (C cells)?

Answer 4

• minority of thyroid cells (<10%)
• involved in control of serum calcium homeostasis
• found outside of follicles
• secrete calcitonin
• this inhibits osteoclasts from resorbing/ breaking down bone resulting in decrease in calcium in the blood
• other tissues also produce calcitonin so replacement is not required in the absence of thyroid gland

Question 5

Describe thyroid hormone structure

Answer 5

• thyroid-based hormone that contain iodine
• T3 contains 3 iodine atoms
• T4 contains 4 iodine atoms
• iodine essential for thyroid hormone biosynthesis (iodine comes from diet e.g. seafood, crops and dairy)

Question 6

How is iodide transported into the thyroid?

Answer 6

• must be transported into the cell from bloodstream
• this is the first, rate-limiting step in biosynthesis
• occurs against concentration gradient (high level of intracellular iodide)
• iodide uptake mediated by sodium/iodide symporter (NIS), located on basolateral cell membrane

Question 7

How does the sodium iodide symporter work?

Answer 7

• iodide transported against conc. gradient by cotransporting iodide with 2 Na+ ions, Na+ goes down conc. gradient and brings I- with it
• dependent on Na/K ATPase
• transports 3 Na out and 2 K in, this sets up Na gradient

Question 8

How is iodide transported into the follicular lumen?

Answer 8

• iodide then moves across and out of cell by passive transport into lumen (site of hormone biosynthesis)
• mediated in part by pendrin, an anion transporter located on the apical/ luminal cell membrane

Question 9

What is thyroglobulin (Tg)?

Answer 9

• thyroid-specific protein
• most highly expressed protein in thyroid gland
• large glycoprotein dimer (660kDa) that is secreted into follicular lumen
• serves as a scaffold for hormone synthesis and storage
• 132 tyrosine residues - up to 1/3 can becomes iodinated (store for iodine) while <10 becomes thyroid hormones

Question 10

What is thyroid peroxidase (TPO)?

Answer 10

• thyroid specific protein
• located on apical/ luminal membrane
• enzyme that catalyses
  
  • oxidation of iodide to facilitate the iodination of Tg tyrosine residues
  • coupling of the iodotyrosines to form thyroid hormone

Question 11

Explain the iodination of thyroglobulin by TPO

Answer 11

• iodide ions oxidised to iodine utilising H2O2 to do this
  (I- + H2O2 –> I)
• TPO catalyses iodination of tyrosine residues with iodine
• becomes monoiodotyrosine (MIT) if attaches to 1 iodine or diiodotyrosine (DIT) if 2 attach

Question 12

How does TPO couple iodotyrosyl residues?

Answer 12

• MIT + DIT = T3
• DIT + DIT = T4
  TPO uses hydrogen peroxide here too

Question 13

What are the final steps of thyroid hormone synthesis?

Answer 13

• iodinated Tg is stored in the follicular lumen as colloid
• when thyroid hormone is required, the colloid is endocytosed from lumen into vesicles
• through fusion of these vesicles with lysosomes, Tg is digested by proteolytic enzymes to release T3 and T4
• T3 and T4 are secreted into the bloodstream via thyroid hormone transporters such as MCT8

Question 14

How is hormone synthesis controlled?

Answer 14

• TSH (from pituitary gland) stimulates thyroid to produce and release thyroid hormone
• pituitary releases TSH in response to TRH (thyrotropin releasing hormone) from hypothalamus
  HPT axis =
  Hypothal. –> Pituitary –> thyroid
  TRH TSH

The thyroid hormones then can generate the -ve feedback loop

Question 15

How does the TSH receptor (TSHR) work?

Answer 15

• TSHR is a G-protein coupled receptor
• located in the basolateral membrane of thyroid cells
• TSH binding causes a conformational change in the TSHR
• TSHR then replaces the GDP on the G protein with a GTP thereby activating it
• activated alpha subunit can then activate adenylyl cyclase which releases cyclic AMP which then bind to PKA to releases active catalytic subunit of PKA
• activated PKA can have effects on gene transcription to form new proteins e.g. Tg and NIS
• almost all effects of TSH on thyroid hormone biosynthesis are mediated by adenylyl cyclase/ cAMO

Question 16

Describe the action of TSH

Answer 16

• stimulates expression of NIS
• also required plasma membrane localisation of NIS
• TSH can stimulate localisation of pendrin to apical membrane
• TSH stimulates expression of Tg
• stimulate expression of TPO
• TSH stimulates endocytosis of colloid

Question 17

What are the thyroid binding proteins?

Answer 17

T3 & T4 are hydrophobic
~70% bound to thyroxine-binding globulin (TBG)
~15-20% bound to transthyretin (TTR)
~10-15% bound to albumin
Free T4 = 0.03%
Free T3 = 0.3%
(unbound (free) hormone enters target cell)

Question 18

How much of the thyroid hormones present are T3 and T4?

Answer 18

T4: T3
total (nmol/L) = 60-150 1.2-2.9
free (pmol/L) = 9-24 4.7-8.2
half life (days) = 5-7 1-3

• majority of secreted thyroid hormone is T4
• T4 is a prohormone
• T3 is a biologically active hormone
• 20% of T3 produced each day is secreted from thyroid
• remaining 80% is generated by conversion of T4

Question 19

How is T4 converted to T3?

Answer 19

• amount of active hormone (T3) is regulated by the peripheral tissues
• thyroid hormone transporters such as MCT8 take up T3 and T4 into the tissues
• T4 is converted by to T3 by enzymes called deiodinases
• T3 passes back into bloodstream - controls serum T3 levels as well as local tissue T3 levels
• the presence of transporters and deiodinases in peripheral tissues represent further levels of hormone regulation

Question 20

Describe the action of deiodinases

Answer 20

• there are 3 enzymes
  D1 = both, can both activate and inactive TH
  D2 = outer ring (main activator of T3), main source of serum T3 and local T3
  D3 = inner ring (inactivates T3 and T4), clearance of T3 and T4 - protective role

D2 removes iodine from outer ring of T4 => active T3

Inactivation mediated by D3 hormone, inactivates T4 to rT3 by taking I from inner ring
- confirmation of having I in outer ring makes rT3 inactive
D3 can also remove inner ring I from T3 => inactive T2

Question 21

Where do the deiodinases act?

Answer 21

D1 = liver, kidney 
D2 = CNS, pituitary, BAT, skeletal muscles, placenta, heart 
D3 = CNS, placenta

Question 22

What are the main roles of thyroid hormones?

Answer 22

• acts on almost all tissues
• development
• regulation of growth
• control of basic metabolic rate

Question 23

How do thyroid hormones affect growth and development?

Answer 23

• humans born without a thyroid gland become severely mentally deficient (cretins)
• TH is essential for early brain development - a lack of hormone causes irreversible motor and cognitive impairment
• lack of hormone is childhood and adolescents results in a short stature and delayed skeletal mutation

Question 24

How do thyroid hormones affect basic metabolic rate (BMR)?

Answer 24

• BMR = the rate at which heat is produced by an individual in a resting state
• thyroid hormone increases BMR through oxygen consumption and heat production
• alters mitochondrial activity, the major site of oxidative processes in the cell
• T3 in XS can increase BMR by 60-100%, in its absence, BMR reduced to 50%

Question 25

What are the effects of T3 on bodily mechanisms?

Answer 25

- increased carbohydrate metabolism (glyconeolysis and gluconeogeneis) - increased fat metabolism by mobilising lipids and circulating free fatty acids - decreased circulating cholesterol and triglycerides by increasing cholesterol secretion in bile - regulates contractility and heart rate

Question 26

How do these effects arise?

Answer 26

- TH binds to nuclear hormone receptors which are ligand (T3)-activated transcription factors - bind to thyroid hormone response elements (TREs) - 2 thyroid hormone receptor genes (alpha and beta); and 2 splice variants of each a1: widely distributed e.g. heart, muscle, BAT a2: widely distributed, cannot bind T3 but does bind response element on gene (therefore may suppress) b1: widely distributed, e.g. liver, kidney, brain, pituitary b2: pituitary and CNS - involved in -ve feedback

Question 27

Describe the action of thyroid hormone

Answer 27

- TH transported into cell by TH transporter (e.g. MCT8) - T4 => T3 - T3 enters cells and binds to TREs of gene promoters - THR heterodimerise with RXR to stimulate gene expression - in absence of T3, THR can still bind to TRE and bind with RXR but there are corepressor complexes to prevent expression, when T3 binds corepressor replaced with coactivator to allow gene expression - T3 responsible for activation of many genes

Question 28

What are the non-genomic effects of T3 and T4?

Answer 28

- rapid effects, mostly at the plasma membrane, that are not blocked by inhibitors of mRNA or protein synthesis - induction of signal transduction pathways e.g. MAPK, cAMP/PKA - modulation of plasma membrane ion pumps or channels - rapid uptake of glucose and amino acids