Thyroid Gland Flashcards
Give a brief overview of thyroid function.
- essential for normal development, especially the CNS and bone
- essential for normal metabolism of many body tissues
What are the functions of the different cells in the thyroid?
- Follicular cells synthesise and secrete thyroid hormone.
- Colloid is where TH is stored.
- C-cells secrete calcitonin - to reduce plasma Ca2+
Thyroid hormones are derived from two iodinated tyrosine molecules.
Describe two thyroid hormones learnt.
T4: major form released to blood, less active (prohormone)
T3: active form, converted in target cells
What are the steps to TH synthesis? PART 1
- Active uptake of I- across the basolateral membrane, against the concentration gradient. Done by the Na/I symporter, and stimulated by TSH.
- Iodide efflux (by diffusion) then occurs across the apical membrane via pendrin exchanger
What are the steps to TH synthesis? PART 2
- At the extracellular apical membrane, iodide is oxidised to iodine and covalently bind to tyrosine residues within thyroglobulin (TG) macromolecules.
- Requires TPO and H2O2.
What are the steps to TH synthesis? PART 3
- Tyrosine residues may be iodinated in one (mono-iodotyrosine, MIT) or two (DIT) positions.
- Coupling of iodotyrosine residues produces T4 (DIT-DIT) and a smaller amount of T3 (DIT-MIT). Catalysed by TPO
What are the steps to TH release? PART 1
- Under influence of TSH, colloid droplets consisting of thyroid hormones within TG molecules are taken back up into the follicular cells by pinocytosis.
- Fusion of the colloid droplets with the lysosomes causes hydrolysis of TG and release of T3 and T4.
How do thyroid hormones circulate?
- Bound to plasma proteins.
- Mainly TBG (~70%), transthyretin (10-20%) and albumin (10-20%).
How do thyroid hormones act on target tissues? PART 1
- TH receptors (TRs).
- Belong to the nuclear receptor superfamily - high affinity for T3
- Ligand-activated transcription factors.
How do thyroid hormones act on target tissues? PART 2
- Activation requires dimerisation with another TR or a retinoid X receptor (RXR).
- TRs are encoded by two genes: TR α and TR β.
Describe the metabolic regulation of THs.
- Done by selenodeiodinases, abbreviated to D1-3.
- Used selenium as their trace element, so it is essential in the diet.
- Regulate the amount of T3 available to bind with the receptor.
Outline the functions of the selenodeiodinases.
- Activate by converting T4 to T3 (D2)
- Inactivate by converting T3 to T2 (D3)
- Inactivate by converting T4 to rT3 (D3).
What are some ways TH increases metabolic rate?
- affects number and size of mitochondria, enzymes, Na/K ATPase activity, etc.
- positive inotropic and chronotropic effects on the heart
- synergises with the sympathetic nervous system
What are some ways TH is involved in energy metabolism?
- antagonises insulin signalling
- gluconeogenesis, lipolysis
Describe how TH moves across the membrane.
- HYPOTHESIS: TH diffused across the cell membrane to work.
- TRUTH: TH transporters are required to allow TH across membrane e.g MCT8.
What genetic mutation is MCT8 linked to?
Allan-Herndon-Dudley Syndrome
Describe the role of the hypothalamic-pituitary-thyroid axis.
- Negative feedback control of thyroid hormone synthesis and secretion
- Hypothalamic neurosecretory cells release TRH into the portal capillaries.
- TRH stimulates thyrotrophs of the anterior pituitary to secrete TSH.
What happens with high levels of T4 and T3?
- Release of TSH and TRH are turned down.
Describe congenital hypothyroidism.
- Inadequate thyroid hormone production in neonates.
- Occur because of anatomic defects in gland, an inborn error of thyroid metabolism, or iodine deficiency.
How is congenital hypothyroidism investigated and treated in neonates?
- Preventable with a simple neonatal heel prick before testing for TSH levels.
- Possible congenital hypothyroidism if the levels of TSH are high.
Describe the TSH receptor and its actions.
- G-protein coupled receptor
- Done through Adenylate Cyclase (αGs and the cAMP-PKA cascade).
- Leads to iodide uptake, the transcription of NIS (Na+/I- symporter), TG and TPO, etc.
What are the actions of TSH?
- increases iodine uptake
- stimulates other reactions involved in TH synthesis (eg. TPO)
- stimulates uptake of colloid
- induces growth of the thyroid gland (which can lead to goitre)
Define hyperthyroidism.
- Occurs when there is TH excess.
- Primary - problem with the thyroid gland
- Secondary - problem with pituitary regulation
Define hypothyroidism.
TH deficiency
