The thyroid

Question 1

where does the thyroid originate from embryologically?

Answer 1

the back of the tongue

Question 2

what joins the two lobes of the thyroid?

Answer 2

a central isthmus

Question 3

how can the thyroid be distinguished from other neck lumps?

Answer 3

by its movement from swallowing

Question 4

what nerve and gland are at risk of damage during thyroid surgery and why?

Answer 4

the recurrent laryngeal nerve lies laterally each side

also the parathyroid gland lies posteriorly

Question 5

what is the blood supply to the thyroid?

Answer 5

inferior and superior thyroid arteries

Question 6

what makes up the thyroid tissue?

Answer 6

Colloid mades up the thyroid

it contains iodinated thyroglobulin which then converted to thyroxine

thyroid also contains neuroendocrine cells (C cells) which secrete calcitonin.

Question 7

what is the physiological function of the thyroid?

Answer 7

Iodinated tyrosine molecules form thyroxine which is T4 and T3.

T4 is the main circulating hormone which is converted peripherally the more potent and shorter acting T3.

Question 8

what are the two main types of thyroid receptor?

Answer 8

TRa and TRB

Question 9

what vertebral level is the thyroid?

Answer 9

c5-t1

Question 10

what hormone levels characterise primary hypothyroidism compared to secondary hypothyroidism?

Answer 10

Primary = problem with thyroid itself

so get reduced circulating T4 and high TSH

Secondary = TSH deficiency, usually the result of pituitary disease.

so have low T4 levels and non elevated TSH.

Question 11

what hormone levels characterise primary hyperthyroidism compared to secondary hyperthyroidism?

Answer 11

Primary = increased T3/T4 levels with suppressed TSH

secondary = TSH is not suppressed and T3/T4 us high. This suggests a TSHoma, thyroid hormone resistance or assay interference.

Question 12

what causes hyperthyroidism/thyrotoxicosis?

Answer 12

• Autoimmune (Graves disease) is the commonest cause due to presence of TSH receptor stimulating antibodies
• usually affects young women and follows a relapse-remitting cause
• nodular disease presents presents older than auto-immune and is caused by autonomous secretion of T3/T4 from either a solid toxic nodule or numerous nodules.
• Thyroiditis is inflammation of the thyroid and can occur as a result of infection, medication e.g amidarone or following childbirth and involves the release of thyroxine.

Question 13

what are the clinical features of hyperthyroidism?

Answer 13

• weight loss with increased appetite
• insomnia
• irritability
• anxiety
• heat intolerance
• palpitations
• tremor
• puritis
• increased bowel frequency and loose motions
• menstrual disturbance and reduced fertility
• resting tachycardia
• lid lag
• hyperreflexia
• warm peripheries
• lid retraction and proptosis in graves

Question 14

what investigations are done for hyperthyroidism?

Answer 14

bloods for elevated fT4 and fT3 with undetectable TSH.

elevated fT3 alone with normal fT4 and suppressed TSH is a T3 taco

Question 15

what investigations are done for hyperthyroidism?

Answer 15

bloods for elevated fT4 and fT3 with undetectable TSH.

elevated fT3 alone with normal fT4 and suppressed TSH is a T3 toxicosis

the presence of elevated fT4 and fT3 with non suppressed TSH is unusual

normal fT4 and fT3 and suppressed TSH have subclinical hyperthyroidism suggesting autonomous thyroid activity

thyroid peroxidase antibodies are non specific markers of auto immune thyroid disease

Question 16

what imaging can be done of the thyroid in hyperthyroidism ?

Answer 16

ultrasound

can help confirm nodular thyroid disease but doesn’t not assess gland activity

can do nuclear imaging e.g iodine uptake isotope scan can help determine functionality

if nodular disease, there is increased uptake only in autonomous nodules, in thyroiditis there is absent uptake and in graves disease there is uniform uptake

Question 17

what is the medical treatment for hyperthyroidism?

Answer 17

medication is first line, with thionamides e.g carbimazole and propylthiouracil which reduce T3 and T4 synthesis. Take 4-6 weeks to normalise results.

NB: Thionamides can cause bone marrow suppression, and if unexplained fever or sore throat occurs, urgent FBC is required to exclude pancytopenia and drug should be stopped if neutrophil count is low. Can also get a rash as a side effect.

Also, women of child bearing age should be on contraception with carbamazepine. if they fall pregnant, switch to propylthiouracil.

Question 18

what is the surgical treatment for hyperthyroidism?

Answer 18

1) Radioactive iodine

administration of single dose that causes ablation to the thyroid. don’t do in pregnancy and may lead to flare up of eye disease in pre existing ophthalmopathy. Can commonly cause hypothyroidism so will need lifelong thyroxine replacement e.g levothyroxine. Post treatment will emit radiation so must avoid children and pregnant women.

2) Surgery
good for people who can’t take radioactive iodine e.g mothers with young children. Complications of surgery include bleeding, infection, damage to recurrent laryngeal nerve and temporary/permanent hypocalcaemia due to hypoparathyroidism.

Question 19

what are the causes of primary hypothyroidism?

Answer 19

• autoimmune Hashimotos thyroiditis
• pregnancy, post natally can be misdiagnosed as post partum depression
• iodine deficiency can cause neonatal hypothyroidism
• genetic defect
• drugs e.g amiodarone and lithium
• iatrogenic e.g surgery/radioiodine

Question 20

what causes secondary hypothyroidism (TSH deficiency)?

Answer 20

caused by TSH deficiency due to hypothalamic pituitary disease.

get low fT4 with non elevated TSH.

Question 21

what are the clinical features of hypothyroidism?

Answer 21

• weight gain
• cold intolerance
• fatigue
• constipation
• bradycardia
• myxoedema
• thickening skin

Question 22

what investigation are done hypothyroidism?

Answer 22

bloods

low fT4 and elevated TSH

in Hashimotos autoimmune thyroiditis thyroid peroxidase TPO antibodies are strongly positive

Question 23

how is hypothyroidism treated?

Answer 23

thyroxine replacement, starting at 50-100ug a day. elderly/IHD patients will be started on a lower dose e.g 25ug.

if persistently elevated, suggests poor compliance or Malabsorption. Suppressed or undetectable TSH suggests over replacement, leading to risk of AF and osteoporosis.

Question 24

what is subclinical hypothyroidism?

Answer 24

refers to normal fT4 and elevated TSH.

asymptomatic patient may not need to be treated.

Guidelines recommend starting thyroxine if TSH >10miU/L even if patients are asymptomatic due to likelihood of progression to frank hypothyroidism.

Question 25

what is are the consequence of thionamides?

Answer 25

can cause agranulocyotisis (bone marrow suppression) and patients should be warned of this potential rare side-effect before commencing treatment.

If unexplained fever or sore throat occurs, an urgent full blood count is required to exclude pancytopaenia, and the drug should be stopped if the neutrophil count is low.

A more common side-effect is generalised rash, which disappears after cessation of the drug.