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47 Cards in this Set
- Front
- Back
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What roles does the thyroid gland have?
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Regulates metabolism
Stimulates oxygen consumption Regulates carbohydrate and lipid metabolism Needed for normal growth and maturation |
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Where does the thyroid gland develop from?
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Invagination of the pharynx
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What connects the two lobes of the thyroid gland?
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Narrow isthmus (not obvs in cat or dog)
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How is the thyroid gland supplied with blood?
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Via cranial carotid artery
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Describe its nerve supply
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Rich sympathetic nerve supply
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What is the capsule?
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The connective tissue that surrounds the thyroid gland
Also found in smaller pieces surrounding the individual follicles |
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Apart from thyroid cells and colloid, what else is present in the gland?
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Patches of lymphocytes
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What are the thyroid follicles composed of?
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Secretory epithelium and colloid - gel-like substance
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What are the principal cells?
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Thyroid follicular cells - produce thyroid hormones and thyroglobulin
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What is the main component of the colloid?
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Thyroglobulin
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Where are the thyroid hormones stored?
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Extracellularly as part of the thyroglobulin in the colloid
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What does an increase in gland activity do?
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Increases the thickness of the secretory cells and reduces the amount of colloid
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What will the principal cells look like if relatively inactive?
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Squamous or low cuboidal
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What will they look like if active?
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Columnar
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Where are the parafollicular cells found?
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In the epithelial lining or in the connective tissue spaces
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What is another name for the parafollicular cells and what do they produce?
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Clear cells / C-cells
Calcitonin |
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Which amino acid is the precursor for thyroid hormones?
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Tyrosine bound covalently to iodine
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What determines its biological activity?
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The position of the iodine
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What forms of thyroid hormone are there?
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T4 - thyroxine
T3 - triiodotyronine Reverse T3 All 2 tyrosines |
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How is thyroglobulin formed?
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Iodination and condensation of tyrosine
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How are the thyroid hormones secreted?
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Colloid is ingested by the principal cells and its peptide bonds hydrolysed into T3 and T4.
The hormones are discharged into capillaries -> central circulation |
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What are the 3 functions of principal cells?
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Collect and transport iodine
Synthesise thyroglobulin Remove thyroid hormones from thyroglobulin and secrete them into the circulation |
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What is the uptake of iodine like?
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Active process against concentration gradient
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How is the iodine incorporated?
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It is taken up by the principal cells as they have "iodine traps" on their plasma membrane and iodide is transported into the follicle lumen along with thyroglobulin
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How is the iodine added to tyrosine?
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By the enzyme thyroid peroxidase
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What are the 2 roles of thyroid peroxidase?
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Iodination of tyrosine
Synthesis of T4 or T3 from two iodotyrosines (coupling reaction) |
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What is the ratio of T4:T3 in the thyroglobulin?
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5:1
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What causes the secretion of the thyroid hormones
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TSH secretion from anterior pituitary
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What does a high concentration of TSH result in?
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Faster rate of endocytosis and thus a release of thyroid hormone release into circulation
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Which has the shortest half life?
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T3
(half lives are longer in man than in dog) |
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What do free thyroid hormones bind to?
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Thyroxine-binding globulin
Plasma proteins Albumin |
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What is the need for the binding globulin?
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It stops loss of thyroid hormone and iodine by urinary filtration
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What are the differences seen in exogenous T4 and T3?
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T4 - slow developing, long-lasting effects (as more is bound)
T3 - fast developing, harder to control effects |
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Is T4 or T3 more active?
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T3 is 5-10 times more active
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What then happens to T4?
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It is deiodinated by 5-monodeiodinase to T3 in target tissues
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What causes a hypothyroid state?
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If there is a conversion failure by the enzyme in the liver and kidney to put T3 into plasma
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How does most T3 and T4 enter cells?
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By active transport - membrane transporters
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How do stimuli affect thyroid hormone secretion?
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Tells thyrotropin releasing hormone neurones to cause secretion
TRH acts on thyrotrophs in anterior pituitary to cause thyroid stimulating hormone release which binds to a TSH receptor |
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How is thyroid activity regulated?
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Negative feedback
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How does thyroid hormone affect metabolism?
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Increases metabolism and heat production (except gonads and brain)
Increases BMR Increases glucose uptake in gut Increases metabolism effects of catecholamines, cortisol, growth hormone, glucagon Hypothyroid causes increased blood cholesterol |
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How does thyroid hormone affect the cardiovascular system?
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Increases responsiveness to catecholamines -> vasodilation
Causes increased contractility of myocardium Increased heart rate, pulse pressure and cardiac output Increases O2 consumption by myocardium |
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How does thyroid hormone affect growth and development?
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Needed for normal growth and development
Required for moulting of hair, feathers, scales, skin Required for amphibian development e.g. tadpole metamorphosis |
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How does thyroid hormone affect the nervous system?
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Needed in foetus and neonate for CNS development
Too little sluggishness, too much anxiety and nervousness Spinal reflexes slowed with hypothyroidism |
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How does thyroid hormone affect the reproductive system?
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Link between hypothyroidism and infertility
Normal reproductive behaviour |
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Why is thyroid hormone needed for hibernation?
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Thyroid gland becomes less active to lower BMR
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What alterations in thyroid hormone are there during pregnancy?
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Increased T4-binding globulin - increases TSH and increases thyroid hormone production
Increased demand for iodine Chorionic gonadotropin stimulates thyroid |
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What causes a goitre?
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Swelling of thyroid gland caused by hypothyroidism
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