Thyroid Flashcards
describe the basic anatomy of the thyroid
- left and right lobe joined by a central isthmus
- RLNs lie laterally on each side
- parathyroid gland lies posteriorly
what substances is thyroid tissue made up of (2)
- colloid - contains iodinated thyroglobulin
- neuroendocrine cells (C-cells which secrete calcitonin
in which condition are calcitonin levels elevated
medullary thyroid cancer
what are thyroid hormones made up of
iodinated tyrosine molecules to form T3/T4
- T4 is the main circulating hormone which is converted peripherally to the more potent and shorter acting T3
what are the 2 types of thyroid receptor
TRα and TRβ
what are the actions of thyroid hormones
- increase BMR
- affect growth in children
- acts on CVS to increase HR, CO & contractility
- body temperature
- digestive tract
- skin, hair, nails
what is the blood supply to the thyroid gland
rich vascular supply from STA and ITA
- STA: 1st branch of ECA which splits into anterior and posterior branches
- ITA: branch of thyrocervical trunk and splits into superior and inferior
explain the physiology of the thyroid axis
- hypothalamus releases TRH
- stimulates anterior pituitary to release TSH
- TSH stimulates thyroid gland to release T3/T4
- when T3/T4 are released, the hypothalamus and anterior pituitary suppress the release of TRH/TSH to lower levels of T3/T4 = negative feedback
what is primary hyperthyroidism
the thyroid behaves abnormally - excessive T3/T4 which suppresses TSH (low)
↑ T3/T4, ↓TSH
what are the main causes of primary hyperthyroidism (4)
- Graves’ disease
- Inflammation (thyroiditis)
- Solitary toxic thyroid nodule
- Toxic multinodular goitre
what is secondary hyperthyroidism
the pituitary behaves abnormally and produces excessive TSH (e.g. pituiatry adenoma) which stimualtes the thyroid to produce excess T3/T4
↑ TSH, T3, T4
what factors may affect the outcome of thyroid functions tests
may be affected by non-thyroidal illness so they are best interpreted when the patients are relatively well rather than during acute illness
- meds e.g. Li, amiodarone
- pregnancy
what is primary hypothyroidism
thyroid produces inadequate thyroid hormones causing inc TSH
↓ T3/T4, ↑ TSH
what are common causes of primary hypothyroidism
- Hashimoto’s thyroiditis
- Iodine deficiency
- Treatment for hyperthyroidism e.g. Li, amiodarone
- Subacute thyroiditis (de Quervain’s)
what is secondary hypothyroidism
the pituitary produces inadequate TSH causing under-stimulation of thyroid gland so insufficient thyroid hormones
↓ TSH, T3, T4
what are causes of secondary hypothryoidism
- pituitary failure
- tumours
- surgery
- RT
- sheehan’s
- also associated with Down’s, Turner’s, coeliac disease
name 3 antibodies associated with the thyroid gland and state what conditions they are associated with
- anti-thyroid peroxidase (anti-TPO): most relevant in autoimmune thyroid disease e.g. Grave’s and Hashimoto’s
- anti-thyroglobulin (anti-Tg): can be present in normal individuals w no pathology but usually raised in Grave’s, Hashimoto’s and thyroid cancer
- ## TSH receptor antibodies: autoantibodies that mimic TSH and bind to the TSH receptor stimulating T3/T4 release - cause Grave’s disease
what imaging technqiues are useful in thyroid function testing
- USS
- radioisotope scans
how are ultrasound scans useful in thyroid function tests
- helps diagnose thryoid nodules
- distinguishes between cystic and solic nodules
- guides a biopsy of thyroid lesion
how do radioisotope scans of the thyroid gland work
- radioactive iodine given orally or IV and travels to thyroid where it is taken up by cells
- iodine is used by the thyroid cells to produce thyroid hormones
- the more active thryoid cells are, the faster the radioactive iodine is taken up
- gamma camera then detects gamma rays emitted from the iodine
- gives functional information of thyroid gland
what are radioisotope scans used to investigate
hyperthryoidism
thyroid cancers
what are the investigations findings of radioisotope scans in different thyroid pathologies
- graves: diffuse high intake
- toxic multinodular goitre/adenoma: focal high uptake
- thyroid cancer: cold areas (abnromally low uptake)
what are general features of thyrotoxicosis
- weight loss
- manic/restlessness
- heat intolerance
what are cardiac features of thyrotoxicosis
- palps, tachycardia
- high-output cardiac failure in elderly pt
how does thyrotoxicosis affect the skin
- increased sweating
- pretibial myxoedema: erythematous, odematous lesions above the lateral malleoli
- thyroid acropachy: clubbing
what is toxic multinodular goitre
aka Plummer’s disease
- nodules develop on the thyroid gland which are unregulates by the thyroid axis
- continously produce excess thyroid hormones
- common in pt >50
what are specific features of Grave’s disease
- exopthalmos
- pretibial myxoedema
- thyroid acropachy
how does pretibial myxoedema arise
glycosaminoglycan deposition under the skin on the anterior leg
- gives skin discoloured, waxy, oedematous apperance
what is the triad of thryoid acropachy
- digital clubbing
- soft tissue swelling of the hands and feet
- periosteal new bone formation
what does thryoid scintigraphy of Grave’s disease show
diffuse, homogenous, increased uptake of radioactive iodine
what are general features of hyperthyroidism
- anxiety/irritability
- sweating/heat intolerance
- tachycardia
- weight loss
what is the typical presentation of thyroiditis
initial period of hyperthyroidism followed by under-activity (hypot)
give 4 causes of thyroiditis
- de quervain’s
- hashimoto’s
- postpartum
- drug-induced
what is de quervain’s thyroiditis and its 3 phases
aka subacute thyroidits - condition causing temp inflamm of thyroid
1. thyrotoxicosis
2. hypothyroidism
3. return to normal
initial thyrotoxic phase involves: excess T3/T4, thyroid swelling, flu-like symptoms, raised CRP/ESR
how is de quervain’s thyroiditis treated
usually self-limiting
- NSAIDs
- B-blockers
- levothyroxine
what is T3-toxicosis
elevated fT3 alone with normal fT4
suppressed TSH
what is thyroid storm
rare but life-threatening complication of thyrotoxicosis
what are precipitating events of thyroid storm
- thyroid or non-thyroidal surgery
- trauma
- infection
- acute iodine load e.g. CT contrast media
what are clinical features of thyroid storm
- fever > 38.5ºC
- tachycardia
- confusion and agitation
- nausea and vomiting
- hypertension
- heart failure
- abnormal liver function test - jaundice may be seen clinically
what additional treatment may be required in thyroid storm
fluid resus
anti-arrhythmics
b-blockers
how is hyperthyroidism managed
- initially give propranolol to block adrenergic effects
- 1st line ATD: thionamides e.g. carbimazole starting at 40mg then titrated down to maintain euthyroidism
- take for **12-18 months **
- once the thyroid hormone levels are normal (4-8 weeks), can continue on maintenance carbimazole
what is a major complication of carbimazole therapy
agranulocytosis (bone marrow suppression)
- unexplained fever or sore throat requires FBC to exclude pancytopenia and stop drug if neutropenic
give an example of a second line ATD
propylthiouracil
- small risk of severe liver reactions so carbimazole preferred
what are definitive treatment options of hyperthyroidism
- radioactive iodine
- thyroidectomy
how does radioactive iodine treatment work
- single dose uptaken by thyroid gland
- emitted radiation destroys proportion of thyroid cells = ↓ hormone production
- remission can take 6 months after which hypothyroidism = long term levothyroxine
what are strict rules that must be followed with radioactive iodine treatment
- women must not be pregnant/breastfeeding and must not get pregnant within 6 months of treatment
- men must not father children within 4 months
- limit contact w people after dose esp children, pregnant women
what is the most common cause of hypothyroidism in the world
iodine deficiency
what treatments used for hyperthyroidism have the potential to cause hypothyroidism
- carbimazole
- propylthiouracil
- radioactive iodine
- thyroid surgery
what are classical features of hypothyroidism
- weight gain
- cold intolerance
- fatigue
- constipation
- bradycardia
what is the management of hypothyroidism
oral levo (synthetic T4 which metabolises to T3 in the body)
-typical starting dose 50-100ug / day
if there is a persistently elevated TSH during thyroxine replacement what does this suggest
- under-replacement
- poor compliance
- malabsorption e.g. from coeliac disease or other meds e.g. Fe, Ca
if there is a suppressed or undetectable TSH during thyroxine replacement what does this suggest
over-replacement –> inc risk of AF and osteoporosis
what is subclinical hypothyroidism
normal fT4 with elevated TSH
