Thyroid Gland Flashcards
What shape is the thyroid gland?
Butterfly shaped
Where is the thyroid gland located?
Just inferior to the larynx
What is the thyroid gland composed of?
Right and left lateral lobes, one on either side of the trachea
What are the lateral lobe connected by?
Isthmus, anterior to the trachea
What is the third small lobe about 50% of thyroid glands have?
Pyramidal lobe, extends superiorly from the isthmus
What is the normal mass of the thyroid?
About 30g
What makes up most of the thyroid gland?
Thyroid follicles
What does the wall of each follicle consist of?
Primarily of cells called follicular cells, most of which extend to the lumen of the follicle
What surrounds each follicle?
Basement membrane
What happens when follicular cells are inactive?
Their shape is low cuboidal to squamous, but under the influence of TSH they become active in secretion and range from cuboidal to low columnar in shape
What two hormones are produced by follicular cells?
Thyroxine (T4- longer half life)
Triiodothyronine (T3- speedy)
What lies between cells?
Parafollicular cells (C cells)
What do C cells produce?
Produce the hormone calcitonin (CT) which helps regulate calcium homeostasis
What are the steps of the synthesis and secretion of T3 and T4?
Iodide trapping Synthesis of thyroglobulin Oxidation of iodide Iodination of tyrosine Coupling of T1 and T2 Pinocytosis and digestion of colloid Secretion of thyroid hormones Transport in the blood
What does the parathyroid gland do?
Produces different hormones
What actions of thyroid hormone?
Lung maturation in utero (surfactants)- easier breathing as lung more compliant
Bone maturation in infants
Neonatal brain development (proliferation, differentiation, synapse development)
Infant growth by enhancing growth hormone
Increased BMR to achieve calorigenesis
Increased in beta-adrenergic receptor quantity dilates resistance arterioles and sensitivity
Increased skeletal muscle contractility
Increased cardiac muscle contractility directly on Ca/Na exchange, increased heart rate, cardiac output, pulse pressure
What is increased inotropic effect lead to?
Greater force of contraction
What does increased chronotropic action lead to?
Greater speed
What does increased lusitropic effect lead to?
Shorter diastolic relaxation time, all beta effects, diastolic pressure goes up
What is the parathyroid hormone?
An 84 a.a. peptide produced by the chief cells of the parathyroid gland
What do parathyroid hormone do?
- Fall in plasma Ca2+ detected by receptors on the parathyroid chief cells
- Parathyroid hormone (PTH) production increased
- PTH increases osteoclastic activity but these have no PTH receptors. Current idea is that o-blasts with PTH receptors recruit o-clasts
What happens when bone is reabsorbed?
Calcium phosphate is set free. Plasma Ca2+ is therefore restored but there is a phosphate “leak”. Phosphate is lost in the urine both by increased filtration loss but also by reduced phosphate reabsorption. In addition, bicarbonate reabsorption is inhibited
What is calcitonin?
A 32 a.a. peptide produced by the parafollicular cells of the thyroid gland: may act to increase calcium deposition in bones but maybe only during periods of physiological stress e.g. pregnancy. No diseases associated with excess or deficient production
What are the steps of calcitonin production?
- Increased plasma Ca2+ detected by receptors on parafollicular cells
- Calcitonin production increased
- Calcitonin decreases osteoclastic activity
