The Thyroid gland

What is the thyroid gland?

Gland in the neck that sits in front of the trachea

• Produces hormones that regulate metabolism

What hormones does the thyroid gland produce?

Synthesises, secretes and stores Tri-iodothyronine and Tetra-iodothyronine (thyroxine)

• Thyroid gland takes in iodine from diet and converts it into T3 and T4

• Only gland to take in iodine

• Increases metabolic rate

• Essential for growth, development and healthy life

Difference between a normal thyroid, hypo and hyper thyroid

Stored in the lumen - colloid

• Thyroid folliclle cells produce thyroglobulin protein and stored in the lumen

How are T3 and T4 made?

• TSH receptors are on follicular cells. Binding causes thyroglobulin production

• Thyroglobulin enters the colloid

• Sodium iodide transporter is used to bring I^- into the cells and then the pendrin transporter to get into the lumen

• Iodide gets oxidised into atomic iodine by thyroperoxidase

• Iodine attaches to tyrosine rings on thyroglobulin to make MIT/DIT

• MIT+DIT= T3, DIT+DIT=T4

• Thyroglobulin is taken into the follicle cells and then cut up into T3 and T4

• These are then secreted into the bloodstream

What are the stages of T3 and T4 secretion?

• Iodine uptake into cell

  • Active transport by carrier

• Iodination of tyrosine residues on thyroglobulin (TG)

  • By thyroperoxidase enzyme

  • Called organic binding

• Formation of T3 and T4 from MIT and DIT

• Endocytosis of TG, enzymatic release of T3 and T4, secretion

What do the thyroid hormones do to metabolism?

• Regulate metabolism in most tissues.

• Modulate effects of other hormones (e.g. glucagon)

• T3 more potent than T4

• Increase metabolism of carbohydrates, proteins and fat

• Increase oxygen use and heat production

• Increased basal metabolic rate

• Deamination, deiodination and conjugation with glucuronic and sulfuric acids

How do thyroid hormones affect growth and development?

• Direct and indirect action on cells

• Potentiates secretion and effects of growth hormone

• Needed for normal skeletal development, and maturation of CNS.

• Act by entering cell nucleus, T3 binds to receptor and switches it off.

• Receptor normally STOPS transcription

• So, T3 causes increase mRNA and protein synthesis in cells

How are thyroid hormones transported?

• T3 and T4 transported bound to a protein carrier, TBG, little free.

  • TBG (thyroxine binding globulin)

• LARGE pool T4, less active and mainly in circulation

• SMALL pool of T3, mainly intracellular

  • T4 converted to T3 in target tissues, T3 is the active form

How is secretion of thyroid hormones controlled?

Negative feedback

• Hypothalamus has a stimulus and releases Thyrotropin releasing hormone

• Positive effect in anterior pituitary gland

• Pituitary releases thyroid stimulating hormone

• Thyroid gland is now stimulated and T3 and T4 are released

What does the hypothalmic pituitary thryoid axis do?

Plays the major role in regulating thyroid hormone homeostasis

• Under the control of neurons in the median region of the PVN that synthesise and release TRH into the the pituitary gland

How is TSH brought to cells?

Rich blood supply of capillaries

What do the parafollicular cells do?

Produces calcitonin