thyroid anatomy, physiology and disease Flashcards by lucy macdonald

what is the nerve innervation of the thyroid gland?

autonomic nerve supply (Parasympathetic from vagus nerves, and sympathetic fibres from superior, middle, and inferior ganglia of the sympathetic trunk (with blood vessels)

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what is the blood supply of the thyroid gland?

Superior and inferior thyroid arteries (branch of external carotid) +/- thyroidea ima

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what is the venous drainage of the thyroid gland?

Three pairs of veins drain- superior/ middle thyroid vein > internal jugular, inferior thyroid vein> brachiocephalic veins

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what completes the negative feedback loop in the thyroid by suppressing production of TSH and TRH?

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what are the 6 steps of thyroid hormone synthesis?

thyroglobulin synthesis
uptake and concentration of iodide (I-)
oxidation of iodide (I-) to iodine (I)
iodination of thyroglobulin
formation of monoiodotyrosine (MIT) and di-iodotyrosine (DIT)
secretion

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what happens when you combine MIT and DIT?

MIT + DIT = T3 (triiodothyronine)
DIT + DIT = T4 (thyroxine)

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is T3 or T4 more potent?

T3 (4 times more potent than T4), it is the major biologically active thyroid hormone

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what type of T3 and T4 can enter cells?

unbound

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what effects does thyroid hormone have on metabolic rate and thermogenesis?

Thyroid hormones increase basal metabolic rate
-Increase number & size of mitochondria
-Increase oxygen use and rates of ATP hydrolysis
-Increase synthesis of respiratory chain enzymes

Thyroid hormones increase thermogenesis
~ 30% temperature regulation due to thyroid hormone thermogenesis

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what metabolic effects does thyroid hormone have?

Carbohydrate metabolism
-inc. blood glucose – due to stimulation of glycogenolysis and gluconeogenesis
-inc. insulin-dependent glucose uptake into cells

Lipid metabolism
-Mobilise fats from adipose tissue
-inc. fatty acid oxidation in tissues

Protein metabolism
-inc. protein synthesis

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what effects does thyroid hormone have on growth and development?

Growth
Growth hormone releasing hormone (GHRH) production & secretion requires thyroid hormones
Glucocorticoid-induced GHRH release also dependent on thyroid hormones (permissive action)
GH/somatomedins require presence of thyroid hormone for activity (permissive action).

Development of foetal & neonatal brain
Myelinogenesis & axonal growth require thyroid hormones

Normal central nervous system activity
Hypothyroidism - slow intellectual functions
Hyperthyroidism – nervousness, hyperkinesis & emotional ability

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what effects do thyroid hormones have on sympathomimetic action?

Permissive Sympathomimetic action
Thyroid hormones increase responsiveness to adrenaline & sympathetic NS neurotransmitter, noradrenaline, by increasing numbers of receptors

cardiovascular responsiveness also increased due to this effect – increased force and rate of contraction of heart

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what are the 3 types of de-iodinase and where are they found?

Type I (D1)
is commonly found in the liver and kidney

Type II (D2) is found in the heart and skeletal muscle, CNS, fat , thyroid, and pituitary

Type III (D3) found in fetal tissue and placenta and brain (except pituitary)

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describe hypothyroidism

Deficiency of thyroid hormones
Primary (gland) failure – may be associated
with enlarged thyroid (goitre)
Secondary to TRH or TSH (no goitre)
Lack of iodine in diet (may be associated with goitre)

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what are the symptoms of hypothyroidism?

Reduced BMR
Slow pulse rate
Fatigue, lethargy, slow response times and mental sluggishness
depression
deep hoarse voice
Cold-intolerance
Tendency to put on weight easily

In adults – Myxoedema – puffy face, hands & feet
Babies - Cretinism – dwarfism & limited mental functioning due to deficiency of thyroid hormones present at birth

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describe hyperthyroidism?

Hyperthyroidism - Grave’s disease
Autoimmune disease – TSH receptor antibodies (TRAB) acts like TSH but unchecked by T3 & T4
Exophthalmos – bulging eyes; Autoimmune inflammation of the extra-ocular muscles
Goitre –enlarged thyroid gland

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what are the symptoms of hyperthyroidism?

Increased BMR
Very fast pulse rate
Increased nervousness and excessively emotional
insomnia
Sweating & heat intolerance
Tendency to lose weight easily

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what is secondary thyroid disease?

hypothalamic or pituitary disease with no thyroid gland pathology

what thyroid hormones do you measure to test for disease?

TSH, free T4 and free T3

what are the hormone levels in primary hypothyroidism?

low free T3 & T4
high TSH

what are the hormone levels in primary hyperthyroidism?

high free T3 & T4
low TSH

what are the hormone levels in secondary hypothyroidism

low free T3 & T4
low (or normal) TSH

what are the hormone levels in secondary hyperthyroidism?

high free T3 & T4
high (or normal) TSH

what is myxoedema?

severe hypothyroidism and is a medical emergency

what is pretibial myxodema?

is a rare clinical sign of Graves’ disease - infiltrative buildup on lower 3rd of legs

what are the 3 causes of primary hypothyroidism?

goitrous (e.g. hashimoto's, iodine deficiency) non-goitrous (e.g. atrophic thyroiditis) self-limiting (e.g. following withdrawal of antithyroid drugs)

what are the causes of secondary hypothyroidism?

Diseases of the hypothalamus and pituitary gland: Infiltrative Infectious Malignant Traumatic Congenital Cranial radiotherapy Drug-induced

what are the causes of secondary hypothyroidism?

Diseases of the hypothalamus and pituitary gland: Infiltrative Infectious Malignant Traumatic Congenital Cranial radiotherapy Drug-induced

what are the skin & hair clinical features of hypothyroidism?

Coarse, sparse hair Dull, expressionless face Periorbital puffiness Pale cool skin that feels doughy to touch Vitiligo may be present Hypercarotenaemia pitting oedema

what is the treatment of hypothyroidism?

gradual restoration of normal metabolic rate to avoid cardiac arrythymias levothyroxine

who does myxoedema coma commonly affect?

Typically affects elderly women with long standing but frequently unrecognized or untreated hypothyroidism

what are the typical findings of myxoedema coma?

ECG: bradycardia, low voltage complexes, varying degrees of heart block, T wave inversion, prolongation of the QT interval Type 2 respiratory failure: hypoxia, hypercarbia, respiratory acidosis Co-existing adrenal failure is present in 10% of patients

how do you treat myxoedema coma?

Intensive care, remember – A, B, C! Passively rewarm: aim for a slow rise in body temperature Cardiac monitoring for arrhythmias Close monitoring of urine output, fluid balance, central venous pressure, blood sugars, oxygenation Broad spectrum antibiotics Thyroxine cautiously (hydrocortisone)

what is the difference between thyrotoxicosis and hyperthyroidism?

thyrotoxicosis is the clinical, physiological and biochemical state arising when the tissues are exposed to excess thyroid hormone Hyperthyroidism refers specifically to conditions in which overactivity of the thyroid gland leads to thyrotoxicosis

what causes of thyrotoxicosis are not associated with hyperthyroidism?

- Thyroid inflammation (thyroiditis) Subacute (de Quervain’s) thyroiditis Post-partum thyroiditis Drug-induced thyroiditis (e.g. amiodarone) - Exogenous thyroid hormones Over-treatment with levothyroxine Thyrotoxicosis factitia - Ectopic thyroid tissue Metastatic thyroid carcinoma Struma ovarii (teratoma containing thyroid tissue)

what are the hormone abnormalities in grave's disease?

low TSH and high T3 & T4 (hyperthyroidism)

describe thyroid bruits

rare clinical sign specific to grave's disease associated only with large goitres hypervascularity of the thyroid auscultate over thyroid

describe thyroid storm

Medical emergency, so… A,B,C! Severe hyperthyroidism Respiratory and cardiac collapse Hyperthermia Exaggerated reflexes May require mechanical ventilation Typically seen in hyperthyroid patients with an acute infection/illness or recent thyroid surgery Treatment: Lugol’s Iodine, glucocorticoids, PTU, β-blockers, fluids, monitoring

what is the 1st line treatment of hyperthyroidism?

carbimazole (TPO inhibitor which blocks thyroid hormone synthesis)

what drug is used as 1st line treatment of hyperthyroidism in the 1st trimester of pregnancy?

propylthiouracil (PTU)

what are the side effects of anti-thyroid drugs?

Generally well tolerated drugs 1-5% will develop allergic type reactions – rash, urticaria, arthralgia Cholestatic jaundice, ↑liver enzymes, fulminant hepatic failure (PTU) agranulocytosis - stop all ATDs

what other types of drugs can be used to treat hyperthyroidism?

beta blockers (propanolol drug of choice) - immediate symptomatic relief of thyrotoxic symptoms radioiodine- 1st line for grave's disease relapse (cant be used in pregnancy), risk of hypothyroidism

what are some causes of thyroiditis?

Hashimoto’s De Quervain’s/subacute (viral) Post-partum Drug-induced (amiodarone, lithium) Radiation Acute suppurative thyroiditis (bacterial)

what are the hormone levels in subclinical thyroid disease

abnormal TSH (high=hypo, low=hyper) with normal T3 & T4