The Thyroid Gland Flashcards
what shape are thyroid hormones?
shaped like a butterfly, lies across trachea at base of larynx, one of larger endocrine glands, (15-20g), but neither visible nor palpable in health.
what are the 2 active forms of thyroid hormones?
T3 (triiodothyronine) and T4 (thyroxine).
what are the 2 cell types of thryoid glands>
C (clear) cells
2. Follicular cells
what are the function of C cells
secrete calcitonin (Ca2+ regulating hormone).
what is the function of follicular cells
support thyroid hormone synthesis and surround hollow follicles.
what are thyroid follicles?
spherical structures whose walls are made of follicular cells
what is the centre of thyroid follicles filled with?
colloid = sticky glycoprotein matrix. Contain 2-3 mths supply of TH.
what do follicular cells manufacture?
the enzymes that make thyroid hormones as well as thyroglobulin, a large protein rich in tyrosine residues.
finish the sentence:
the enzymes and thyroglobin are packaged into vesicles and exported from the foliicular cells into the…
colloid
follicular cells actively concentrate …. from the plasma and transport it into the collide
iodide
what happens to the concentrated iodided that is transported into the colloid?
combines with the tyrosine residues to form the thyroid hormones
wht are tyrosine and iodide derived from?
diet
how doess iodide enter the follicular cells from the plasma via?
Na/I transporteer
how is iodide then transported into the colloid?
pendrin transporter
what happens in response to TSH?
portions of the colloid are taken back up into the follicular cell by endocytosis. Within the cells they form vesicles which contain proteolytic enzymes that cut the thyroglobulin to release thyroid hormones.
are T3 and T4 lipid soluble or water soluble?
lipid soluble
what is the significance of T3 and T4 being lipid soluble?
pass across the follicular cell membrane into the plasma where they bind to plasma proteins, mainly thyroxine-binding globulin. Transporter proteins may also be involved in this process as rare mutations in this protein cause major disruption to TH balance.
where do T3 and T4 circulate?
in the plasma
how is the movement of TH from colloid to plasma triggered/controlled?
under the influence of TSH released from the pituitary
what does TSH stimulate?
the follicular cells to endocytose colloidal thyroglobulin. When not stimulated, the thyroid hormones are stored in the colloid.
does thyroxine binding globulin have a high or low affinity for T4?
high which means it releases it slow into the plasma
is it more T3 or T4 that circulates in the form of protein bound T4?
T4
90% of TH binding to TH receptors inside cells is …
T3
the TH receptor has a much higher affinity for
T3 than T4
how is T4 deionised to T3?
by deiodinase enzymes
what are the functions of thyroid hormone>
raises metabolic rate and promotes thermogenesis, typically through promoting futile cycles of simultaneous catabolism and anabolism.
increase hepatic gluconeogenesis, although no effect on BG providing pancreas is releasing adequate insulin (therefore not a counter regulatory hormone)
net increase in proteolysis
net increase in lipolysis
critical for growth (lack of TH results in retarded growth)
essential for brain development in utero (maternal iodine deficiency = congenital hypothyroidism/cretinism)
what are the causes of hyperthyroidism?
Graves Disease (common) - antibodies produced that bind mimic TSH and continually activate the thyroid gland. Increased release of TH switches off TSH release from anterior pituitary so [TSH]plasma very low. Thyroid gland may be 2-3x normal size due to hyperplasia. Hyperactivity of cells also apparent.
Thyroid Adenoma (rare ) - hormone-secreting thyroid tumour
what are the symptoms of hyperthyroidism?
. Increased metabolic rate and heat production
weight loss/ heat intolerance
2. Increased protein catabolism
muscle weakness/weight loss
3. Altered nervous system function
hyperexcitable reflexes and psychological disturbances
4. Elevated cardiovascular function.TH is permissive to epinephrine, receptors
increased HR/contractile force, high output, cardiac failure
what are causes of hypothyroidism?
Hashimoto’s Disease - autoimmune attack of thyroid gland
Deficiency in dietary iodine – only 50mg/year(!) required but many areas of the world soil has insufficient quantities. Main source of dietary iodine was table salt which was enriched with iodine. But no longer in the UK! Milk, Fish, seafood and seaweed are good sources.
Idiopathic – no known cause, may be linked to thyroiditis.
what are symptoms of hypothyroidism
Decreased metabolic rate and heat production
weight gain/cold intolerance
- Disrupted protein synthesis
brittle nails/thin skin - Altered nervous system function
slow speech/reflexes, fatigue - Reduced cardiovascular function
slow heart rate/weaker pulse
