Thyroid and parathyroid gland 6,7,8 Flashcards

1
Q

Briefly describe the thyroid

A

Endocrine gland

Secretes T3 and T4

Located in neck Two lobes united by a narrow isthmus

Containing follicles C cells- involved in control of serum calcium homeostasis, secrete calcitonin, inhibit osteoclasts from resorbing bone

Follicular endothelial cells- majority of thyroid cells, line follicular lumen filled with colloid, controlled by TSH, responsilbe for production of T3 and T4

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2
Q

Describe thyroid hormone synthesis

A

Thyroid hormones contain iodine (essential for thyroid hormone biosynthesis - sources include fish wheat)

Triiodothyronine (T3) has 3 iodine atoms

Thyroxine (T4) has 4 iodine atoms

Iodine transport from the blood to cell- first rate limiting step Active transport via Na-I symporter (NIS) located on the basolateral cell membrane mediates iodine uptake and can also let Na into the cell

Na conc gradient maintained by Na/K ATPase letting Na out and K in

Iodine transport into follicular lumen (out of the cell) via pendrin- passive

Thyroglobulin- most highly expressed protein in the thyroid gland serves as a scaffold for hormone synthesis

Thyroid peroxidase- located on apical membrane, catalyses oxidation of iodide, iodination of thyroglobulin tyrosine residues, and the coupling of the iodotyrosines to form thyroid hormone MIT +DIT➡ T3 DIT+DIT➡ T4

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3
Q

Describe thyroid hormone release

A

Stored in follicular lumen as colloid

When thyroid hormone is requied, the colloid is endocytosed from lumen into vesicles

Fusion of these vesicles with lysomes causes the digestion of thyroglobulin to release T3&T4 Secreted into bloodstream via MCT8 transporters

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4
Q

Describe thyroid hormone regulation

A

Hypothalamus- TRH➡ pituitary- TSH➡ thyroid- T3&T4➡ negative feedback (reduce levels of TSH and TRH)

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5
Q

Describe the TSH receptor

A

G-protein coupled receptor

Located in the basolateral membrane of the thyroid cells

Conformation change in TSHR G protein after TSH binding replaces GDP with GTP. Activating it

Intracellular effects (activation) mediated by cAMP➡ PKA➡ CREB+ CBP increase transcription of proteins such as Thyroglobulin and NIS

(PKA phosphoylates transcription factors such as CREB and CBP which activate gene expression)

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6
Q

Action of TSH

A

TSH binds to receptor and stimulates signalling cascade. Can increase NIS and NA/I symporter to increase iodie uptake, increased about of NIS on membrane. TSH can stimulate the amount of pendrin in plasma membrane where it mediates iodide crossing apical membrane, can express Tg protein and stimulate expression of TPO by stimulating the oxidaition, iodination and conjugation of thyroid hormones. TSH can stimulate endocytosis of colloid back into cell.

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7
Q

Describe the transport of thyroid hormones

A

T3 and T4 are Hydrophobic and so bound to thyroid new binding globulin (TBG) (70%), transthyretin (TTR)(15%-20%) and albumin (10%-15%)

Only free hormone enters the cell

Most of circulating thyroid hormones are T4, a prohormone, T3 is the active hormone

T3 is active hormone

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8
Q

Describe the conversion of T4 to T3

A

MCT8 take up T3 and T4 into the tissues. T4 Converted to T3 by deiodinases in peripheral tissues. T3 passes back into blood stream.

T4 = iodine redidues on inner and outer ring

T3 = one iodine on outer ring

As a further level of hormone regulation D1 works on both inner and outer rings- can activate and inactive thyroid hormones- found in liver and kidney

D2 works on the outer ring, removing iodine from outer ring- main activator of T3, in CNS, pituitary, skeletal muscle, placenta, heart

D3 works on the inner ring- inactivates thyroid hormones- CNS and placenta

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9
Q

Describe thyroid hormone action

A

Control growth and development- essential for early brain development,

Lack of hormone results in mental retardation and delayed skeletal maturation

Basic metabolic rate- increase heat produced through increases oxygen consumption and heat production- alters mitochondrial activity

Other- increase carbohydrate, fat metabolism, decreases circulating cholesterol and triglycerides by increasing cholesterol secretions in bile

Regulates cardiac contractility and heart rate

Acts on nuclear hormone receptors Ligand (T3)-activated transcription factors bind to thyroid hormone response elements (TRE)

TRE = 2 genes with 2 splice variants

  • Alpha 1- heart and muscle
  • Alpha 2- cannot bind T3 but can bind response element on gene- suppressive?
  • Beta1- liver, kidney, Brain, pituitary
  • Beta2- pituitary and CNS- involved in negative feedback

Non-genomic effects- cytoplasmic signals transduction pathways- MAPK, cAMP and PKA Modulate of PM ion pumps or channels Rapid uptake of glucose and amino acids

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10
Q

Describe hyperthyroidism

A

Decreased serum TSH, increased serum T3 and T4. Hypothyroidism is the opposite

Graves hyperthyroidism- autoimmunity- most common

Toxic nodular goitre

TSH-R antibodies can cross placenta and pass to baby. But baby will make antibodies against them so will eventually dissapear

Thyroiditis- inflammation

Symptoms- tachycardia, atrial fibrillation, shortness of breath, ankle swelling Weight loss, diarrhoea, increased appetite Tremor, myopathy, anxiety Sore gritty eyes, double vision, stating eyes, itching

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11
Q

Describe Graves disease

A

60-80% of hyperthyroidism

Most common autoimmune disease

Pathogenic stimulating auto-antibodues to TSH receptor on thyroid follicular cells. This causes TSH to be low so no negative feedback occurs, so overexpression of T3 and T4

Eye lid kag, conjunctival oedema, periorbital puffiness, bulging, weakness of eye muscles

Diagnosis by iodine uptake scan TPO abs and TSH abs

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12
Q

Describe treatment of hyperthyroidism

A

Anti thyroid drugs- thionamides- carbimazole, propylthiouracil- blocks iodine incorporation SE- rash, joint pain, sickness, no WBCs, liver disease- propulthiouracil Low cure rate

Surgical removal of thyroid- used for patients with large goitre, in pregnant women, and in severe eye swelling and protrusion

Radioactive iodine- 85% cure, may lead to hypothyroidism, not for pregnant women, (cancer and infertility?)

Treatment aims to relieve symptoms, restore T4/T3

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13
Q

Describe hypothyroidism

A

Hashimotos thyroiditis- autoimmune antibodies for TPO and thyroglobulin- genetic predisposition After treatment for hyperthyroidism,

iodine deficiency- major cause of hypothyroidism

Congential - cretinism Sympotoms- Bradycardia, heart failure, pericardial effusion Myxoedema, rash on kegs, weight gain, constipation, depression, psychosis T

reated using Levothyroxine to normalise T3 and T4 levels

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14
Q

Describe thyroid cancer

A

Rare Assessment of thyroid function- serum TSH, thyroid abs, calcitonin/thyroglobin measurement.

Goiture = swelling on neck due to swelling of thyroid gland

Assessment of thyroid size- X-ray thoracic inlet, CT or MRI of neck, respiratory loop

Assessment of thyroid pathology- radionuclide, ultrasound scanning Used to differentiate between benign cystic nodules and tumours, guidance for fine needle aspiration

Treatment- radiation, chemo, TK inhinbitor, surgery, levothyroxine

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15
Q

Describe calcium homeostasis

A

Bone- main reservoir for calcium

Bone turnover- calcitonin

Soluble calcium-

Kidneys, GI tract and bone all contribute to calcium in circulation

Functions of skeleton: support and muscle attachments, protection for vital organs and marrow, ion homeostasis for calcium and phosphate

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16
Q

What is the importance of the parathyroid glands

A

4 parathyroid glands that sit ontop of thyroid gland

Regulate calcium and phosphate levels

Secrete parathyroid hormone (PHT) in response to low calcium and high phosphate

PTH increase calcium reabsorption in the DCT and absorption in the intestine and increases calcium release from the bone via osteoclast activity

Decreases phosphate reabsorption

PHT also increases release of Vitamin D, which increases reabsortion of calcium and phosphate within diet

17
Q

Describe parathyroid hormone

A

84aa but biologically active in the first 34aa

Half life 8mins

Normal adult range 1.6-6.9 pmol/l Binds to G protein coupled recpetors in kidney (DCT for reabsorption and stimulates production of active vit D (1,25(OH)2D)) and osteoblasts- bone reabsorption

Negative feedback- 1,25D3 (vitamin D) inhibits PTH transcription PTH (mRNA translation) is inhibited by increased serum calcium

AS calcium levels rise, the receptors on parathyrod cells will detect this, stopping production of PTH

18
Q

Describe the importance of vitamin D in calcium homeostasis

A

Precursor- 25-hydroxyvitamin D

Vitamin D2- plant origin (ergocalciferol)

D3- animal origin (cholecalciferol)

1,25 dihydroxyvitamin D binds to the vitamin D receptor (VDR) an intracellular receptor (active form, the more hydroxylations, the more active it is)

From diet eggs and fish amd made in skin

7-dehydrocholesterol transported to liver Normal 7.5-50nmol/l

19
Q

Decribe calcitonin

A

Produced by thyroid c-cells

Released in hypercalcemia,

inhibits bone reabsorption of calcium by osteoclasts

Non-essential to life

20
Q

Describe fibroblast growth factor 23 (FGF23)

A

Produced by osteocytes

Released in response to high serum PO4

Increases renal excretion of PO4 (phosphate) and suppresses renal synthesis of active vit D

Main inducer of FGF23 is 1,25D3 (active vit D)

FGF23 is a marker of kidny disease

21
Q

Describe the basic structure of bone

A

Epiphysis (growth plate)

metaphysis- contain cancellous/trabecular bone

Diaphysis- cortical/compact/lamellar

Articular cartilage

Growth plate it where formation occurs

22
Q

What is bone made of?

A

ECM which is able to calcify Collagen fibres with preferential orientation and non-collagenous protein essential to bone- osteocalcin, osteonectin, osteopontin

Calcification occurs with formation of hydroxyapatite crystals

Cells: Osteocytes- embedded in calcified bone matrix

Osteoblasts- bone forming cells produce matrix constituents and aid calcification (originate form MSCs)

Osteoclasts- bone resorbing cells found in contact with calcified bone surface in lacunae Multinucleated- originate from bone marrow lineage Produce acid and enzymes to resorb mineral and matrix

23
Q

Describe hyperparathyroidism

A

Parathyroid glands dont work

Raised serum PTH

Primary- parathyroid tumour causes hypercalcaemia and low serum phosphate

Secondary- renal disease, Kidneys loose ability to retain phosphate in body due to decreased urinary phospahte nd increased excretion of it. Cant make the active form of Vit D, causes decrease in calcium and increase in phosphate which causes increase in PTH.

Treatment with phosphate binders (reduce phosphate in serum) or vitamin D analougues (to decrease level of PTH)

Tertiary- long standing secondary leads tro irreversible parathyroid hyperplasia

24
Q

Describe rickets/osteomalacia

A

Active vit D deficiency, lack of mineralizarion of collagen component of bone.

Rickets- children- bow legs (growth plate is weak)

Osteomalacia- adults- pseudo fractures.

Treatment = vitamin D replacement

25
Q

Describe osteoporosis

A

Loss of bone mass/density

Aging - curvature of spine (humpback)

PostMenopause- decline of oestrogen which is involved in bone turnover

Treatment- hormone replacement, bisphosphonates (block osteoclasts), denosumab (RANK ligand antibody - blocks interaction between osteoclasts and osteoblasts so stops activity), intermittent PTH

All treatments Reduces bone remodelling