Thyroid

Karina and Hope

Anatomy What is the blood supply to the thyroid gland?

Arteries: Superior thyroid artery (external carotid), Inferior thyroid artery (thyrocervical trunk) and thyroid ima.Veins: superior, middle thyroid veins (into IJV) and inferior thyroid veins (brachiocephalic vein)

What is its embryological origin? (Basic)…Pharyngeal arch (1st). Descends down the thyroglossal ducts which then disappear.

NB: The thyroid moves on swallowing because it is in the pre-tracheal fascia.

Functions• What three hormones does the thyroid gland

secrete? – T3 (tri-iodothyroxine)– T4 (thyroxine) – Calcitonin

• What do they do? – T3/T4:

• Metabolism (increases BMR and O2 consumption)• Maturation & differentiation• Neurological function (synapse formation, myelinogenesis

and neuronal outgrowth)• Growth

– Calcitonin: • Reduces calcium in the blood

Cretinism

Histology

• Where is calcitonin produced? – Parafollicular C cells

• What happens in the follicle cell? – Thyroglobulin synthesis– Iodine trapping

• What is colloid?– Solution containing

thyroglobulin + T3/T4

2

3

1

What are the 6 steps of thyroid hormone synthesis?

1. Thyroglobulin synthesis2. Uptake and concentration

of iodide (I-)3. Oxidation of iodide (I-) to

iodine (I)4. Iodination of thyroglobulin5. Formation of MIT and DIT6. Secretion

Thyroglobulin synthesis

Uptake and concentration of iodide (I-)

Oxidation of iodide (I-) to iodine (I)

Iodination of thyroglobulin

Formation of MIT and DIT

Secretion

Blood

What are the 6 steps in thyroid hormone synthesis?

1. Thyroglobulin

synthesis

NIS2.I-Pendrin

Colloid3.

I0I-

TPO

I I I I I II

I I I I I II

MITDIT

DIT DIT

4.

5. MITDIT

DIT DIT

T3

T4

T3

T4

Follicle cells

6.

I-

I-

I-

I-

I-

I-

I-

Questions on the production of thyroid hormone…

• What is thyroglobulin? – Binding protein produced by the follicular cell. Contains tyrosine

residues which iodine binds to. Forming T3/T4.• What side of the follicular cell would you find pendrin and NIS?

– Basal NIS– Apical: pendrin

• What enzyme oxidises iodine to iodide?– TPO (thyroid peroxidase)

• T4 = ? + ? – T4 = DIT + DIT

• T3 = ? + ?– T3 = MIT + DIT

Outline the hypothalamo-pituitary-thyroid (HPT) axis

What else is secreted from the anterior pituitary? FSHLHACTHTSHGHProlactin

Thyroid hormones in the blood

• How do T4 and T3 travel in the blood?– Lipophilic - cannot dissolve in the blood. – Must be associated with proteins. • 70% thyroxin binding blobulin (TBG) • 30% bound to albumin.

• Only the free T3/T4 can enter the cell.

How does thyroid hormone have an effect on target cells?

• T3/T4 have to enter cell via transporter MCT8• In order to have an effect - T4 must be converted

to T3.• There are 3 deiodinases which convert T4 to T3.– 1. Cell surface: T4--> T3 cell surface of most cells– 2. Intracellular: T4 --> T3 CNS and pituitary– 3. Extracellular: T4 --> rT3 (reverse) CNS and placenta

• Activation of receptors T3Ralpha and beta causes new protein synthesis via transcription and translation.

Hyperthyroidism Hypothyroidism

Symptom sorter…Hyperthyroidism Hypothyroidism

Heat Tolerance Slow/ lethargic

Weight and muscle loss Overweight /obese

Increased appetite Alopecia (loss of lateral third of eyebrow)

Diarrhoea Goitre

Nervous irritability

Goitre

What is this symptom?

1. Iodine deficiency (low levels of T4: induces TSH secretion

2. Graves disease (high levels of T4) autoimmune disease that produces thyroid stimulating immunoglobulin - acts as TSH

3. Tumours (benign or cancer)

What are three causes of goitre?

• Explain why goitre forms in Graves disease…

Thyroid is overstimulated by TSI (thyroid stimulating immunoglobulin), therefore the gland increases in size in response in order to release more thyroid hormone.

How do both low and high T4 induce goitre?

TREATMENT

Hyper

Drugs Carbimazole

Radioactive iodine

Surgery

Hypo Drugs Levothyroxine

What are the risks associated with thyroidectomy? (think Tunstall)External branch of superior laryngeal branch is at risk.Parathyroid gland damageBleeding

What condition in the hand is associated with a thyroid problem?Carpal tunnel (hypothyroidism)

Treatment For hyperthyroidism?

For hypothyroidism?