Thyroid physiology, TFTs & hypothyroidism flashcards
18 high-yield MRCP Part 1 cards from the Intellect Medicos Endocrinology & Diabetes course. Try to answer each question before you open it — then study them properly with spaced repetition, which brings each card back just before you would forget it.
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Which enzyme oxidises iodide and iodinates thyroglobulin in the thyroid follicular cell?
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Thyroid peroxidase (TPO).
Why? Iodinated tyrosines form MIT and DIT, which couple to make T3 (MIT + DIT) and T4 (DIT + DIT).
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Which enzyme converts T4 to the more active T3 in peripheral tissues?
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5′-deiodinase (mainly in liver, kidney and muscle).
Why? T4 is largely a prohormone; T3 binds nuclear receptors in target cells.
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In pregnancy or on the combined pill, why does total T4 rise while free T4 stays normal?
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Oestrogen increases thyroxine-binding globulin (TBG), raising bound (total) hormone only.
Why? Only free hormone is biologically active, which is why free T4 and TSH are measured.
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What thyroid function pattern is typical of sick euthyroid syndrome?
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Low T3, low or low-normal T4, and low or normal TSH.
Why? Acute illness reduces binding proteins, peripheral T4→T3 conversion and pituitary TSH output. Repeat TFTs after recovery rather than treating.
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Why may TSH be low in the first trimester of a normal pregnancy?
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hCG has TSH-like activity and stimulates the thyroid.
Why? The effect is greatest with high hCG, e.g. hyperemesis gravidarum; free T4 is usually normal or mildly raised.
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What TFT pattern suggests central (secondary) hypothyroidism?
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Low free T4 with a low or inappropriately normal TSH.
Why? The pituitary fails to respond to low thyroxine. Check other pituitary hormones, especially cortisol, before starting levothyroxine.
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Raised free T4 and T3 with a normal or high TSH suggests which two conditions?
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A TSH-secreting pituitary adenoma or thyroid hormone resistance.
Why? In primary hyperthyroidism TSH should be fully suppressed; a non-suppressed TSH points centrally or to receptor resistance.
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Which antibodies are characteristic of Hashimoto's thyroiditis?
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Anti-thyroid peroxidase (anti-TPO) antibodies, often with anti-thyroglobulin antibodies.
Why? Hashimoto's is the commonest cause of hypothyroidism in iodine-sufficient areas.
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Which thyroid malignancy is associated with Hashimoto's thyroiditis?
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Primary thyroid lymphoma.
Why? Chronic autoimmune lymphocytic infiltration predisposes to lymphoma (usually B-cell). Suspect it with a rapidly enlarging goitre.
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What is the usual treatment of Hashimoto's encephalopathy?
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Corticosteroids.
Why? It is a rare autoimmune encephalopathy (confusion, myoclonus, ataxia) with high anti-thyroid antibodies; thyroid function may be normal.
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Which syndrome combines goitre with congenital sensorineural deafness?
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Pendred syndrome.
Why? It is autosomal recessive, with defective iodine organification; patients are euthyroid or mildly hypothyroid.
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In myxoedema coma, which drug is given empirically before or with thyroid hormone?
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IV hydrocortisone (100 mg 8-hourly).
Why? Adrenal insufficiency often co-exists; giving thyroid hormone first raises cortisol demand and can precipitate adrenal crisis.
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How should a hypothermic patient with myxoedema coma be rewarmed?
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Slowly (passive rewarming).
Why? Rapid rewarming causes peripheral vasodilatation and can precipitate shock.
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Name two drugs that commonly cause hypothyroidism.
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Amiodarone and lithium (also interferon).
Why? Amiodarone's large iodine load suppresses thyroid hormone synthesis (Wolff–Chaikoff effect), especially with underlying autoimmune thyroiditis; lithium inhibits thyroid hormone release.
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In a patient taking amiodarone, which thyroid test is the most reliable marker of thyroid status?
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TSH.
Why? Amiodarone blocks T4→T3 conversion, so free T4 may be raised even when the patient is euthyroid.
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A woman on levothyroxine becomes pregnant. How should her dose change?
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Increase it early (by about 25–50%) as soon as pregnancy is confirmed, and check TSH each trimester.
Why? Thyroxine requirements rise early in pregnancy, and maternal hypothyroidism harms fetal neurodevelopment. Recheck TSH 6–8 weeks postpartum; breastfeeding is safe.
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Which commonly co-prescribed tablets reduce levothyroxine absorption?
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Calcium and iron supplements (also antacids) — separate doses by about 4 hours.
Why? They bind levothyroxine in the gut and reduce absorption, which can raise the TSH.
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For subclinical hypothyroidism, at what TSH does NICE suggest considering levothyroxine?
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TSH 10 mU/L or higher on two occasions 3 months apart.
Why? Below this, NICE suggests considering a 6-month trial of levothyroxine only in adults under 65 who have symptoms.
More Endocrinology & Diabetes topics
- ADH, SIADH & diabetes insipidus
- Cushing's & adrenal insufficiency
- Aldosterone, CAH & phaeochromocytoma
- Hyperthyroidism & thyroid cancer
- Pituitary: prolactin, GH & hypopituitarism
- Reproductive endocrinology
- Diabetes: types, pregnancy & complications
- Diabetes drugs & NICE pathways
- DKA, HHS & hypoglycaemia
- Calcium, parathyroid & MEN
- Lipid disorders