Session 13 ILOs - The Thyroid Gland and thyroid disorders Flashcards
Describe the location of the thyroid gland
Location:
• In the neck, lies against and around front larynx and trachea
• Below thyroid cartilage (Adam’s apple)
• Isthmus extends from 2nd to 3rd rings of the trachea
Is the Thyroid gland normally seen? Describe this
- Only visible or palpable when enlarged (goitre) (insert image)
Describe the structure of the thyroid gland
Structure:
- Butterfly shape with two lateral lobes joined by a central isthmus
- Usually 2-3cm across and is one of the largest endocrine glands in the body
- Two major cell types are found in the gland
1. Follicular cells - arranged in numerous follicles separated by connective tissue
2. Parafollicular (C cells) - found in the connective tissue
Describe the chemical structure of the thyroid hormones
T3 - 2 Tyrosine molecules linked together with iodine at 3 positions on the aromatic rings
Monoiodotyrosine + Diiodotyrosine = Triiodothyronine
T4 - 2 Tyrosine molecules linked together with iodine at 4 positions on the aromatic rings
Diiodotyrosine + Diiodotyrosine = Thyroxine/tetraiodothyronine
Which 3 hormones does the thyroid gland produce and where are they produced?
Thyroid gland produces 3 hormones
- Thyroxine (T4) - produced in the follicular cells
- Triiodothyronine (T3) - produced in the follicular cells
- Calcitonin produced in the parafollicular cells.
Describe the production of thyroid hormones (not the process of production)
Production:
- Occurs in the thyroid follicles:
- Thyroglobulin acts as a Scaffold on which thyroid hormones are formed
- Thyroid peroxidase catalyses production of thyroid hormones
Describe the mechanisms of thyroid hormone (T3 and T4) production
Production process:
1) Transport of iodide into the epithelial cells against a concentration gradient.
2) Synthesis of thyroglobulin in the epithelial cells.
3) Exocytosis of thyroglobulin into the lumen of the follicle
4) Thyroid peroxidase regulates:
• Oxidation of iodide to iodine (requires H2O2)
• Addition of iodine to tyrosine acceptor residues on thyroglobulin to form MIT (mono-iodotyrosine) and DIT (di-iodotyrosine).
• Coupling of DIT with MIT or DIT with DIT to form T3 & T4 respectively within the thyroglobulin protein.
T3 & T4 residues are produced in the ratio of ~1:10
Describe the mechanisms of thyroid hormone (T3 and T4) storage
Storage:
- T3 & T4 are stored extracellularly in the lumen of the follicles as part of the thyroglobulin molecules
- Considerable amounts are usually stored and would last for several months at normal rates of secretion
Describe the mechanisms of thyroid hormone (T3 and T4) secretion
Secretion:
- Thyroglobulin is taken into the epithelial cells from the lumen of the follicles by endocytosis.
- Here proteolytic cleavage of the thyroglobulin occurs to release T3 & T4 and these diffuse from the epithelial cells into the circulation.
- T3 and T4 are transported in the blood bound to the protein thyroxine binding globulin
Describe how the activity of the thyroid gland is controlled
- Via negative feedback
1) Hypothalamus releases Thyrotropin releasing hormone TRH - (control hormone) and it travels to anterior pituitary
2) Stimulates anterior pituitary to release Thyroid stimulating hormone (TSH) which travels to thyroid gland
3) This stimulates thyroid gland to release thyroid hormone
4)Thyroid home has its effects on target tissues to affect metabolism and growth and development
Negative feedback aspect:
1) Thyroid hormone feeds back to anterior pituitary and hypothalamus by negative feedback to reduce TSH and TRH secretion
2) TSH is feeding back to hypothalamus to reduce TRH
Describe the effects of thyroid hormones on the body as a whole
Main general effects (interlinked):
- Metabolism
- Growth and development
- Increase basal metabolic rate and heat production (in most tissues, except brain, spleen, testis) (increase number and size of mitochondria in cell and increase synthesis of respiratory chain enzymes)
- Stimulation of metabolic pathways (generally catabolic pathways increase lipid and carbohydrate metabolism)
- Sympathomimetic effects (increase response to catecholamines)
Describe the effects of thyroid hormones on cells/tissues
Tissue specific effects:
- CVS - Increases heart’s responsiveness to catecholamines (noradrenaline and adrenaline)(increase CO and peripheral vasodilation)
- NS- Increases myelination of nerves and development of neurones
Describe the consequences of under-secretion of thyroid hormones
Undersecretion of thyroid hormones - hypothyroidism:
- Symptoms: Obesity, lethargy, intolerance to cold, bradycardia, alopecia, dry skin, constipation (7)
- Low T3
- Low T4
- Elevated TSH
-Children = Cretinism
- Adults = Myxedema
- Hashimoto’s disease can lead to hypothyroidism - autoimmune disease in which the body destroys thyroid follicles - goitre may or may not be present and is 5x more common in women than in men
Describe the consequences of over-secretion of thyroid hormones
Over secretion of thyroid hormone - hyperthyroidism:
- Symptoms: Weight loss, irritability, heat intolerance, tachycardia, increased bowel movements, fatigue or weakness
Also Graves’ disease - autoimmune disease in which the body produces TSI (thyroid stimulating immunoglobulin) can cause Hyperthyroidism
Goitre (enlargement of thyroid gland)
Exophthalamos (bulging eyes)
Describe the features of myxedema
Describe the features of cretinism
