Medicine — Thyroid Disease, NMC MBBS licence examination syllabus (Nepal Medical Council).
Thyroid disease: one feedback loop explains most of it
Learn why TSH moves the wrong way, and the interpretation stops being memory.
Thyroid disease is unusually well suited to being reasoned rather than memorised. There is one hormone axis, it runs on negative feedback, and almost every test result you will be asked to interpret follows from understanding that loop. The symptoms, similarly, sit on a single axis: metabolism is running too slow or too fast, and the features arrange themselves accordingly.
What makes it a common source of error is one counterintuitive fact. In disease of the thyroid gland itself, TSH moves in the opposite direction to the thyroid hormone — so the patient with a failing gland has a high TSH. Students who memorise result patterns without the loop reliably get this backwards under exam pressure.
🩺 Where this lives: Iodine deficiency remains the leading preventable cause of hypothyroidism and of intellectual disability worldwide, and it is a public health problem rather than a clinical one — addressed by salt iodisation programmes rather than by treating individuals. Nepal's mountainous terrain historically made deficiency common. The clinical consequence worth carrying is that maternal thyroid hormone is required for fetal brain development in early pregnancy, which is why hypothyroidism in a pregnant woman is treated with urgency rather than at leisure.
💡 A note on numbers. This chapter gives no doses and no reference ranges. Levothyroxine and antithyroid drug dosing is individualised and titrated against repeat testing. Laboratory reference ranges are assay-specific and differ between laboratories — and in pregnancy, trimester-specific ranges apply. Learn the direction of change here; take the numbers from the report in front of you.
The feedback loop
HOW TO READ ANY THYROID FUNCTION TEST
Ask ONE question first: does TSH move OPPOSITE to T4, or
WITH it?
OPPOSITE → the problem is the THYROID GLAND (primary)
TSH high, T4 low → primary HYPOthyroidism
TSH low, T4 high → primary HYPERthyroidism
TOGETHER → the problem is the PITUITARY (secondary)
TSH low, T4 low → secondary hypothyroidism
A failing pituitary cannot raise TSH even though the
thyroid hormone is low. Rare — but the classic exam
trap, because the low TSH looks superficially like
hyperthyroidism.
SUBCLINICAL disease — the abnormality is in TSH ALONE:
TSH high, T4 NORMAL → subclinical hypothyroidism
TSH low, T4 NORMAL → subclinical hyperthyroidism
The pituitary is exquisitely sensitive, so TSH shifts
before the hormone leaves the reference range. Whether
to treat depends on the degree, symptoms, antibodies,
age and pregnancy — follow guideline, not reflex.
WHY TSH IS THE BEST SCREENING TEST: because that
sensitivity means it changes first. But it is unreliable in
suspected pituitary disease and in acute severe illness,
where thyroid tests are best deferred until recovery.
Symptoms
Both conditions are mistaken for psychiatric illness, and in opposite directions — hypothyroidism mimics depression (and, in the elderly, dementia), while hyperthyroidism mimics an anxiety disorder. This is precisely why the depression chapter insists on excluding hypothyroidism, and why the anxiety section insists on excluding thyrotoxicosis. A single blood test settles both, which makes it one of the highest-value tests in general practice.
Causes
💡 Exam angle: the eye signs settle the cause. Proptosis, lid retraction, diplopia and gritty eyes occur in Graves' disease and not in a toxic multinodular goitre or a toxic adenoma — so a thyrotoxic patient with eye disease has Graves', without needing a scan. Note also that Graves' eye disease runs a course of its own: it can appear before, during or after the biochemical illness, and it does not necessarily improve when the thyroid function is corrected.
A NOTE ON TREATMENT PRINCIPLES
HYPOTHYROIDISM — replacement with levothyroxine, titrated
against repeat TSH after an interval long enough for a
new steady state. Start low and go slow in the elderly
and in ischaemic heart disease: raising the metabolic
rate abruptly increases myocardial oxygen demand and can
precipitate angina.
HYPERTHYROIDISM — three options, each with a trade-off:
ANTITHYROID DRUGS — non-destructive; relapse is common
after stopping. Warn every patient about AGRANULO-
CYTOSIS: a sore throat or fever means stop the drug
and get an urgent full blood count.
RADIOIODINE — effective; usually leads eventually
to hypothyroidism. CONTRAINDICATED in pregnancy and
breastfeeding, and needs caution in active eye disease.
SURGERY — for large goitres, compression, or
when other options are unsuitable. Risks include
recurrent laryngeal nerve injury and hypoparathyroidism.
BETA-BLOCKERS control the adrenergic symptoms — tremor,
palpitations, anxiety — while definitive treatment takes
effect. They do not treat the thyroid.
Doses and durations follow national guidance.
The emergencies
Both emergencies share a structure worth noticing: each is usually precipitated by something else — infection, surgery, stopping medication, cold, or sedative drugs — and treating the thyroid alone without finding that precipitant leaves half the problem in place. Both also have to be treated on clinical suspicion, because thyroid function results will not return in time to be useful.
Pregnancy and the thyroid nodule
Clinical reasoning: four presentations
🔍 Case 1 — low TSH that is not thyrotoxicosis
PresentationA woman with fatigue, cold intolerance and amenorrhoea has TSH low and free T4 low. Because the TSH is low, hyperthyroidism is suspected and an antithyroid drug is considered.
TrapReading TSH in isolation.
ReasoningTSH and T4 are moving together, which means the pituitary is at fault, not the thyroid. A failing pituitary cannot raise TSH despite a low thyroid hormone. Her symptoms are hypothyroid, and the amenorrhoea suggests other pituitary axes are involved too.
AnswerInvestigate for hypopituitarism, assess the other pituitary hormones and image the pituitary. Assess and replace cortisol before thyroid hormone — starting thyroxine first in undiagnosed adrenal insufficiency can precipitate an adrenal crisis.
🔍 Case 2 — the elderly patient who is "just anxious"
PresentationA 74-year-old has new atrial fibrillation, weight loss and low mood. He is withdrawn rather than agitated, so an anxiety state and malignancy are considered.
Key cluesNew atrial fibrillation with unexplained weight loss.
ReasoningHyperthyroidism in older patients often presents apathetically — without the classic restlessness — and may show up mainly as atrial fibrillation, weight loss and low mood. It is a readily missed and readily treated cause of new AF.
AnswerCheck thyroid function in any patient with new atrial fibrillation. Treat the thyrotoxicosis, control rate, and consider anticoagulation per stroke-risk assessment.
🔍 Case 3 — sore throat on an antithyroid drug
PresentationA woman taking an antithyroid drug for Graves' disease develops fever and a severe sore throat. She is given an antibiotic and told to continue her usual medication.
The concernAgranulocytosis.
ReasoningAntithyroid drugs can cause agranulocytosis, and fever with a sore throat is the classic presenting complaint. Treating it as a routine infection without a blood count misses a potentially fatal complication.
AnswerStop the drug immediately and obtain an urgent full blood count. Every patient starting an antithyroid drug must be warned to do exactly this.
🔍 Case 4 — hypothyroidism and a positive pregnancy test
PresentationA woman on long-term levothyroxine discovers she is pregnant. She feels well and plans to continue the same dose until her next routine review in four months.
Key pointRequirements rise in pregnancy.
ReasoningMaternal thyroid hormone is essential for fetal brain development in early pregnancy, before the fetal thyroid functions. Levothyroxine requirements increase, and waiting months for review risks a period of undertreatment at precisely the wrong time.
AnswerContact her clinician promptly for early testing and dose adjustment, with frequent monitoring using trimester-specific reference ranges. Radioiodine would be contraindicated here.
Commonly confused
Confusion
The distinction
Why it matters
Low TSH means hyperthyroid
Check T4 — together means pituitary disease
The classic misreading of thyroid tests.
Primary vs secondary disease
Opposite directions versus same direction
Completely different investigation.
Subclinical vs overt
TSH abnormal with normal T4
Treatment is a guideline decision, not a reflex.
Graves' vs other thyrotoxicosis
Eye signs occur only in Graves'
Names the cause without a scan.
Hypothyroidism vs depression
A blood test separates them
An easily treatable mimic.
Hyperthyroidism vs anxiety
Weight loss, AF, tremor, heat intolerance
Same reasoning, opposite direction.
Typical vs apathetic thyrotoxicosis
Older patients may be withdrawn, not restless
Presents as AF and weight loss instead.
Sore throat vs agranulocytosis
On an antithyroid drug, assume the latter
Stop the drug and check the count.
Rapid revision
MUST-KNOW FACTS
1. In PRIMARY thyroid disease TSH moves OPPOSITE to T4.
2. Primary hypothyroidism: TSH HIGH, T4 LOW.
3. Primary hyperthyroidism: TSH LOW, T4 HIGH.
4. TSH and T4 moving TOGETHER means PITUITARY disease.
5. Secondary hypothyroidism: TSH low AND T4 low.
6. Subclinical disease: TSH abnormal, T4 NORMAL.
7. TSH is the best screening test because it changes first.
8. Thyroid tests are unreliable in acute severe illness — defer.
9. Hypothyroid = everything SLOW; hyperthyroid = everything FAST.
10. Hypothyroidism: cold intolerance, weight gain, constipation, bradycardia.
11. SLOW-RELAXING reflexes are characteristic of hypothyroidism.
12. Hyperthyroidism: heat intolerance, weight loss, diarrhoea, tremor.
13. Hypothyroidism mimics DEPRESSION and, in the elderly, dementia.
14. Hyperthyroidism mimics ANXIETY.
15. Check thyroid function in any new ATRIAL FIBRILLATION.
16. APATHETIC thyrotoxicosis in the elderly looks withdrawn, not restless.
17. Commonest hypothyroidism: autoimmune (Hashimoto's); globally, IODINE DEFICIENCY.
18. Commonest hyperthyroidism: GRAVES' DISEASE.
19. EYE SIGNS occur in Graves' only — they name the cause.
20. Graves' eye disease may precede, accompany or follow the biochemical illness.
21. Pretibial myxoedema and acropachy are also Graves'-specific.
22. Levothyroxine: start LOW and SLOW in the elderly and in ischaemic heart disease.
23. Antithyroid drugs can cause AGRANULOCYTOSIS.
24. Sore throat or fever on an antithyroid drug: STOP and check the blood count.
25. Radioiodine is CONTRAINDICATED in pregnancy and breastfeeding.
26. Beta-blockers control symptoms but do not treat the thyroid.
27. THYROID STORM: fever, tachycardia or AF, delirium, vomiting.
28. Thyroid storm is treated on clinical suspicion — do not await results.
29. MYXOEDEMA COMA: hypothermia, bradycardia, reduced consciousness.
30. Hyponatraemia and hypoglycaemia accompany myxoedema coma.
31. Both emergencies need the PRECIPITANT found and treated.
32. Maternal thyroid hormone is essential for FETAL BRAIN DEVELOPMENT.
33. Levothyroxine requirements RISE in pregnancy — test early.
34. In hypopituitarism, replace CORTISOL BEFORE thyroid hormone.
35. Thyroid nodule: ultrasound and FINE-NEEDLE ASPIRATION CYTOLOGY.
36. Nodule red flags: rapid growth, hard fixed lump, hoarseness, nodes.
💡 Exam angle: the highest-yield single skill is reading a thyroid function test by asking whether TSH and T4 move in the same or opposite directions. The second is spotting Graves' from the eyes. The third is the trio of safety points that questions love — agranulocytosis on antithyroid drugs, radioiodine in pregnancy, and cortisol before thyroxine in pituitary failure.
Syllabus points
The feedback loop and why TSH moves opposite
Reading any thyroid function test
Secondary (pituitary) disease
Subclinical thyroid disease
Symptoms as a single slow-fast axis
Psychiatric mimics in both directions
Causes, and iodine deficiency globally
Why eye signs mean Graves' disease
Treatment principles and their trade-offs
Agranulocytosis on antithyroid drugs
Thyroid storm and myxoedema coma
Thyroid disease in pregnancy
Assessing a thyroid nodule
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