Thyroid and parathyroid endocrinology Flashcards
What connects the two lobes of the thyroid gland?
the isthmus
What cell types does the thyroid gland consist of?
follicular and parafollicular cells
What hormones does the thyroid gland secrete?
T4-thyroxine
T3-thriiodothyronine
calcitonin
Describe the process of thyroid hormone synthesis
- Iodide uptake in thyrocytes by the sodium-iodide symporter (NIS)
- Oxidised by thyroperoxidase (TPO)
- Incorporated into selected tyrosyl residues of thyroglobin (TG)
- results in the formation of mono and diiodotyrosines
- Iodotyrosines are subsequently coupled by TPO to form thyroxine (T4) and triiodothyronine (T3)
- Hydrolysed by lysosomes and T4 and T3 are released into the bloodstream
What is T4?
thyroxine
What is T3?
triiodothyronine
What do T3 and T4 bind to in the blood?
thyroxine-binding globulin (TBG)
Which out of T3 and T4 is more active?
T3 is more active
What is T4 converted into in the peripheral tissues?
T3
What are the effects of thyroid hormones?
Increase BMR
Increase body temperature
Increase appetite
promotes metabolism of nutrients
Which type of cells secrete calcitonin?
C cells
Which cells store thyroid hormone?
colloid cells
What is made in the hypothalamus that causes the release of thyroid stimulating hormone from the anterior pituitary?
thyrotropin releasing hormone
Where do T4 and T3 cause negative feedback?
the hypothalamus
the anterior pituitary
What does TSH stimulate by binding to TSH receptors in the thyroid gland?
uptake of I- by Na/I- symporter
Which enzyme is important in the synthesis of thyroid hormones?
TPO thyro-peroxidase
Which out of T3 and T4 is biologically active?
T3
Describe what serum TSH, free T3 and free T4 would look like in a case of hyperthyroidism?
reduced serum TSH
increased serum free T4
increased serum free T3
What does Grave’s disease cause?
hyperthyroidism
What is the issue if
high serum TSH
low serum free T4
low serum free T3
hypothyroidism
What hormone is secreted by the parathyroid glands?
parathyroid hormone PTH
What does low calcium trigger the secretion of by the parathyroid glands?
PTH
What does high phosphate trigger the secretion of by parathyroid glands?
PTH
What triggers the secretion of parathyroid hormone?
low calcium
high phosphate
Describe the action of PTH
increases calcium absorption in the renal distal tubule
Increases intestinal calcium absorption
Increases calcium release from bones
decreases phosphate reabsorption
How does PTH affect phosphate reabsorption?
decreases phosphate reabsorption
What percentage of calcium is bound to albumin?
50%
What type of cell does PTH stimulate in bone?
osteoclasts
What hormone is produced by the parathyroid glands and stimulates the production of the active form of vitamin D?
PTH
What affect does increased PTH secretion have on urinary phosphate levels?
increases urinary phosphate levels
What effect does increased PTH secretion have on urinary calcium levels?
decreased
What effect does PTH have on bone resorption?
PTH increases bone resorption
What does increased serum calcium inhibit the secretion of?
PTH
What hormone is released by the thyroid gland in response to high calcium?
calcitonin
What is the function of calcitonin?
inhibits bone resorption
