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Chapter: Medicine Study Notes : Endocrine and Electrolytes


TRH (hypothalamus) -> TSH (anterior pituitary) -> T3 and T4 (thyroxine -> peripherally converted to T3)





·        Clinical

·        Biochemical

·        The gland itself: enlarged, smooth, nodules, tender, etc

·        Specific features: eg autoimmune (exophthalmos) or pituitary disease




·        TRH (hypothalamus) ® TSH (anterior pituitary) ® T3 and T4 (thyroxine ® peripherally converted to T3)

·        T3 & T4 stored in thyroid follicles as thyroglobulin

·        T3 exerts greater negative feedback on the pituitary.  Only takes a 10% rise in fT4 to suppress TSH

·        T3 is considerably more metabolically active (ie potent) than T4

·        Liver converts T4 ® T3, as does kidney and muscle

·        T4 goes down first in hypothyroid.  Only measure T3 in hyperthyroid (as it drives symptoms)

·        Bound in plasma to thyroid binding globulin

·        Intercurrent illness: fT4 rises (liver stops converting T4 to T3 straight away – want to be catabolic) then T4 falls to subnormal levels as thyroid production slows. FT3 falls from onset. TSH slowly falls to subnormal levels with severe illness. (ie similar pattern to hypopituitarism)




·        Normal gland weighs about 30 g

·        Nodular or diffuse?

·        Can be hyper, eu, or hypo-thyroid

·        Check size, shape, consistency, mobility

·        Check for dysphagia, stridor, laryngeal nerve palsy (especially multinodular)


Thyroid Tests


·        Free T4 and T3

·        Plasma T4 (= bound T4 + free T4). False high in pregnancy, oestrogens (­TBG). False lows in NSAID, phenytoin, steroids, TBG deficiency

·        Plasma TSH: Newer sensitive test means low levels can be measured Þ don‟t do TRH anymore

·        If low T3 and T4 and normal TSHÞ ?pituitary failure

·        Thyroid isotope scanning: to look for hot spots or cold spots

·        Thyroid antibodies: raised in Hashimoto‟s and some Graves

·        TRH test: Inject to test thyroid. If minimal increase in TSH then: Hyperthyroidism, multinodular goitre, thyroxine replacement, euthyroid Graves disease, autonomous thyroid nodule


Thyroid imaging


·        Can use nuclear medicine, ultrasound (little routine use – can guide FNA), CT (not for intra-thyroid lesions but demonstrates extension and mass effects) or MRI. Only when suspicion of significant pathology

·        Before scanning thyroid (with 99MTc pertechnetate), stop Iodine supplements (eg kelp), thyroxine, and angiography (contrast contains Iodine). Gland must be „hungry‟

·        Normal uptake is 1 - 3 %.  If it takes up too much then hyperthyroid

·        If diffuse uptake then Grave's.  If multinodular then:


o   Cold nodules: 80% are cysts or regressive nodules.  20% are malignant.  ® FNA

o   Hot nodules: maybe with decompensation in rest of gland (¯uptake due to ¯TSH)


Thyroid Eye Disease


·        Retro-orbital inflammation and lymphocyte infiltration. May ® optic nerve compression (® colour desaturation and ¯acuity). Does not parallel degree of toxicosis

·        At presentation, patient may be euthyroid, hypothyroid or hyperthyroid

·        Management: Early referral. Check for keratitis with Rose Bengal eye-drops. Lubricant eye drops. Systemic steroids. Surgical and other treatments


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Medicine Study Notes : Endocrine and Electrolytes : Thyroid |

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