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Thyroid
Organisation of the Body
| Question | Answer |
|---|---|
| Anatomy of the thyroid | Sits just in front of the trachea beneath the thyroid cartilage of larynx Isthmus lies anterior to 2-4 tracheal rings Enclosed in fascial which anchors thyroid to the trachea Profuse blood supply and venous drainage |
| Thyroid relationship to laryngeal nerve | Laryngeal nerve runs posterior to thyroid to supply laryngeal muscles Risk of damage in thyroid surgery and vocal cord paresis |
| Thyroid development | Forms in the midline of the floor of the mouth between the 1str and 3rd branchial arch Downgrowth of the developing tongue around week 4 Descent to final position by week 7 Growth is marked after week 12 when TSH secretion begins Thyroglossal cysts |
| Parathyroid glands | Release parathyroid hormone - control of plasma calcium PTH is released in response to fall in serum calcium to restore ca plasma to normal Parathyroid adenomas secreting excess PTH causes increased risk of bone fractures and kidney stones |
| Development of parathyroid glands | Inferior parathyroid glands are formed from the 3rd pharyngeal pouch and descend together Separate and stop migrating on the thyroid posterior surface Superior parathyroids develop from the 4th pouch |
| Unique features of the thyroid | T3 and T4 are iodothyronine hormones containing iodine Contains many iodide transporters Stores iodine within hormone precursor thyroglobulin extracellularly |
| Storage of prohormones | Thyroid follicle containing thyroglobulin Single layer of follicular cells Produce and secrete T4 in response to TSH Parafollicular cells secrete calcitonin |
| Cellular arrangement in the thyroid | Epithelial follicular cells form follicles The lumen is filled with colloid - thyroglobin precursor Calcitonin acts to lower elevated plasma calcium, calcitonin secreting tumours have no effect |
| Effects of TSH | Stimulates iodide carriers to increase uptake Na K pump increases Na gradient Pendrin moves iodide into lumen TPO converts this to iodine Iodinated thyroglobulin is endocytosed and combined with lysosomes to form T3 and T4 |
| Structure of follicular cells | Fenestrated capillary Lots of RER, granules and lysosomes Microvilli Columnar appearance when producing T4 Cuboidal appearance when not producing T4 |
| Regulation of T3 and T4 production | Hypothalamus secretes TRH in response to cold stress etc Anterior pituitary then secretes TSH Thyroid then secretes T3 and T4 Target tissues increase metabolism, increase body temp and stimulate normal growth |
| T4 | A prohormone than needs to be cleaved by deiodinases to form T3 Converted to T3 by Type 1 deiodinase in liver and kidney and Type 2 in brain, pituitary and adipose tissue Type 3 breaks down T4 and T3 to rT3 |
| Thyroid receptors | Genomic actions - TR in the nucleus bind DNA and activate transcription Timescale - hours Mutations cause Thyroid resistance Works alongside retinoic acid and coregulators |
| What is resting energy expenditure | The minimum calorific requirement needed to sustain life in a resting individual The amount of energy your body would burn if you slept all day A major component of total energy expenditure |
| Role of T3 | Increases resting energy expenditure Acts on nearly every tissue Increases oxygen use and heat production Increases production of Na K ATP ase |
| Effects of T3 on metabolism | Protein metabolism - stimulates protein breakdown Carbohydrate metabolism - potentiates glycogenolysis and gluconeogenesis Lipid metabolism - stimulates cholesterol breakdown and enhances lipolysis |
| Effects of T3 | Increases cardiac output, rate and force Acts by increasing production of myosin, B1 receptors and Ca ATPase Bounding pulse in hyperthyroidism, weak pulse in hypothyroidism Stimulates gut motility Increases muscle activity |
| Developmental effects of T3 | Essential for postnatal growth of the CNS, stimulates the production of myelin, neurotransmitters, axonal growth Stimulates linear growth of bone Frogs with no thyroid do not metamorphosise - remain as tadpoles |
| Hypothyroidism | Apathy Goitre Muscle weakness Constipation Cold hands Intolerance of cold Weight gain Heart slowing and slow pulse Low T4 High TSH |
| Prenatal thyroid deficiency | Poor neural development, intellectual disability Stunted growth Muscle weakness Treated by giving T4 at birth Could potentially treat via generation of functional thyroid from embryonic stem cells - so far only in mica |
| Iodine deficiency | Most common cause of preventable mental retardation and brain damage Common in desert and mountain regions In the UK many pregnant mothers are iodine deficient Food fortification e.g. of salts can be used to prevent this |
| Effects of iodine deficiency | Causes thyroid growth Causes increased secretion of TRH and TSH because of lack of T£/T4 feedback Known as goitre Growth increases iodine trapping capacity of pituitary Pituitary removal causes thyroid atrophy |
| Causes of hypothyroidism | Failure of thyroid growth Lack of iodine Autoimmune inhibition Failure of pituitary to produce TSH or TRH Thyroid hormone resistance Failure of thyroid hormone transporters |
| Hyperthyroidism | Restless Goitre Eye problems Tachycardia Lose weight Intolerance of heat Hot hands Hand tremor Diarrhoea Hight T3 and T4 Low TSH |
| Graves disease | Autoimmune antibodies bind and activate TSH receptor TSH R stimulates enlargement of thyroid gland T3 and T4 very high Treated with radioactive iodine |
| Thyroid cancers | Different types Tumour tissue may comprise functional follicles that secrete T3 or T4 Tumours can also be non-functioning |
| Environmental effects on the thyroid | Several chemicals in pesticides, cigarette smoke etc have been shown to alter thyroid hormone production EDCs measured in umbilical cord blood are associated with altered T4 and TSH in heel prick test |
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