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Thyroid function tests for NEET PG: read TSH and free T4 as a pair

MedicinePublished 27 October 2026 · Recallab

TSH is the pituitary's opinion of the thyroid. Free T4 is the thyroid's output. Read them together, and most thyroid results explain themselves.

A failing thyroid drives the TSH up; an overactive one switches it off. The questions that catch candidates are the results that break this rule: a low thyroxine with a TSH that is not high, a high thyroxine with a TSH that is not suppressed, and results that change when nothing about the gland has changed. These six illustrated Recallab questions work through those patterns, for NEET PG and INI-CET revision. Try each question before opening its worked answer.

Before you name the problem: three questions

Do TSH and free T4 move in opposite directions? If they do, the pituitary is answering normally, and the cause lies in the gland or in something acting on it; if they do not, look above the gland, at the pituitary and its feedback.
Could something other than the gland explain it? Pregnancy, recovery from illness, a supplement or a new tablet can each move the numbers.
Is the result physiologically possible in this patient? A very high thyroxine in a calm woman with a normal pulse points to the assay, not the gland.

Read TSH and free T4 as a pair

Recallab example 1 · a low T4 with a TSH that is not high

Thyroid function results from four patients are drawn in the figure. One of them also has a large pituitary tumour. Which patient is that? Thyroid-stimulating hormone and free thyroxine for four patients, printed beside the normal ranges.

Look first: decide for each patient whether the TSH is answering the thyroxine correctly, then find the one where it is not.

A chart card with four columns headed Patient 1 to Patient 4 and two rows headed TSH and Free T4, with the normal ranges in a narrow column at the right. Patient 1: TSH 38 mIU/L, free T4 5 pmol/L. Patient 2: TSH less than 0.01 mIU/L, free T4 48 pmol/L. Patient 3: TSH 0.6 mIU/L, free T4 6 pmol/L. Patient 4: TSH 7.5 mIU/L, free T4 14 pmol/L. Normal ranges: TSH 0.4 to 4.0 mIU/L, free T4 10 to 22 pmol/L. — Recallab question 55854. Four patients' TSH and free thyroxine, beside the normal ranges.

APatient 2
BPatient 4
CPatient 1
DPatient 3

Show the answer and explanation

Answer: D. Patient 3

D. Patient 3. Patient 3 has a low free thyroxine with a TSH inside the normal range. A failing thyroid would drive the TSH high; here the pituitary cannot respond. That is central hypothyroidism, which fits a large pituitary tumour. The same tumour may have cut corticotropin, leaving hidden cortisol deficiency, and thyroxine speeds cortisol clearance, so cortisol is checked and replaced before thyroxine is started. Treatment is then monitored by free T4, not TSH.

Why the other options lose: Patient 2 (A), with a suppressed TSH and a high thyroxine, is primary thyrotoxicosis, the pituitary correctly switched off by the excess. Patient 4 (B), with a raised TSH and a normal thyroxine, is subclinical primary hypothyroidism, not pituitary failure. Patient 1 (C), with a very high TSH and a low thyroxine, is primary thyroid failure, with the pituitary working hard.

Recallab question 55854 · Medicine › Endocrinology · reference: Harrison's Principles of Internal Medicine

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When something outside the gland moves the TSH

Recallab example 2 · a suppressed TSH at ten weeks

A woman at ten weeks with severe vomiting has a suppressed thyrotropin and a slightly raised free thyroxine, with no goitre and no eye signs. What is the explanation?

Look first: think about which hormone peaks at ten weeks, and what it shares with thyrotropin.

AGraves disease
BGestational transient thyrotoxicosis
CSubacute thyroiditis
DLaboratory error

Show the answer and explanation

Answer: B. Gestational transient thyrotoxicosis

A left panel shows the pregnancy hormone and thyrotropin side by side, with identical alpha subunits and different beta subunits, and the pregnancy hormone binding weakly to a thyrotropin receptor on a thyroid cell. A right panel plots the pregnancy hormone rising to a peak around ten weeks and then falling, free thyroxine following it in a smaller rise, and thyrotropin dipping below the reference band and returning. A box lists goitre, eye signs and antibodies as absent. — Recallab question 53995. A shared subunit, and a rise that comes and goes, from the question's explanation.

B. Gestational transient thyrotoxicosis. Human chorionic gonadotropin shares an identical alpha subunit with thyrotropin and stimulates its receptor. Its level peaks at around ten weeks, when the effect is greatest, and it is commoner and more marked in hyperemesis and in multiple pregnancy. The absence of goitre, eye signs and antibodies distinguishes it from Graves disease. It resolves as the hormone falls after the first trimester, so no antithyroid drug is needed and the tests are repeated after twenty weeks.

Why the other options lose: Graves disease (A) does not fit: there is no goitre, no eye signs, and the picture is transient. Subacute thyroiditis (C) gives a painful, tender gland, and painless thyroiditis usually follows delivery; she has neither. Laboratory error (D) is wrong because this is a recognised physiological pattern.

Recallab question 53995 · Obstetrics › Early Pregnancy Complications · reference: Williams Obstetrics

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Recallab example 3 · a raised TSH after pneumonia

A 59-year-old woman recovering from severe pneumonia in Chandigarh has TSH 8 mIU/L with normal free T4. TSH was low during intensive care, while pre-admission tests were normal. She has no goitre or specific thyroid symptoms but remains tired. Six weeks later TSH normalises untreated. What best explains this course?

Look first: follow the TSH through three stages: before the illness, in intensive care, and in recovery.

APermanent primary hypothyroidism
BRecovery-phase TSH rebound
CPersistent central hypothyroidism
DAutonomous TSH secretion

Show the answer and explanation

Answer: B. Recovery-phase TSH rebound

A time-course schematic with a solid curve rising from below a shaded band to a peak and settling within the band; Acute illness marks the low first segment, TSH overshoot marks the peak, and Spontaneous normalisation marks the final plateau. A nearly level dashed curve lies below. — Recallab question 74240. TSH falls in severe illness and briefly overshoots in recovery, from the question's explanation.

B. Recovery-phase TSH rebound. Severe systemic illness alters thyroid hormone metabolism and hypothalamic-pituitary signalling, so TSH may fall during the acute phase without any destructive thyroid disease. During convalescence, pituitary secretion can briefly overshoot its usual reference interval. A normal free T4 and spontaneous resolution argue against true thyroid hormone deficiency, and tiredness after pneumonia does not establish hypothyroidism. Repeating the tests after recovery separates this transient pattern from persistent thyroid dysfunction.

Why the other options lose: Permanent primary hypothyroidism (A) would not usually normalise without replacement. Persistent central hypothyroidism (C) features a low free T4 with an inadequate TSH, unlike the normal hormone level here. Autonomous TSH secretion (D) keeps the TSH inappropriately high alongside raised thyroid hormones, rather than giving a transient rise in recovery.

Recallab question 74240 · Medicine › Endocrinology · reference: Harrison's Principles of Internal Medicine

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A high thyroxine that is not thyrotoxicosis

Recallab example 4 · a high T4 and a mother with the same results

A woman of 26 has palpitations and a small, smooth goitre. Free T4 is 3.1 ng/dL (0.8-1.8) and free T3 is raised, yet TSH is 3.4 mIU/L (0.4-4.0). Her mother has had identical results for years and is well. Pituitary MRI, alpha-subunit and sex hormone-binding globulin are normal. What is the most appropriate management?

Look first: ask why the TSH is not suppressed, then ask what happens to the tissues if the hormone level is lowered.

AAtenolol alone
BCarbimazole
CTranssphenoidal surgery
DRadioiodine ablation

Show the answer and explanation

Answer: A. Atenolol alone

Left: a feedback loop with the pituitary above the thyroid, the hormone arrow returning to a pituitary receptor drawn as a socket the hormone does not fit, so the TSH arrow stays thick. Right: an upper box, crossed out in red, shows a tablet shrinking the thyroid with the TSH arrow rising and the goitre growing; a lower box with a green tick shows a heart and a small tablet. — Recallab question 70003. A feedback loop that cannot switch off, and why the gland is left alone, from the question's explanation.

A. Atenolol alone. Rising thyroid hormone normally suppresses TSH through T3 receptors in the pituitary. A high free T4 and T3 with an unsuppressed TSH mean that feedback is failing. A normal pituitary MRI and alpha-subunit, and an affected mother, point to inherited resistance to thyroid hormone. The high levels are the body's compensation for resistant tissues, so they must not be lowered; only the palpitations are treated, and a beta-blocker settles them.

Why the other options lose: Carbimazole (B) lowers hormone output, so the TSH rises further and the goitre grows. Transsphenoidal surgery (C) is for a TSH-secreting adenoma, which shows on MRI and usually raises the alpha-subunit; both are normal here. Radioiodine ablation (D) treats Graves disease, where TSH is suppressed; here it would remove compensating hormone and leave her hypothyroid.

Recallab question 70003 · Medicine › Endocrinology · reference: Harrison's Principles of Internal Medicine

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Recallab example 5 · impossible results in a well woman

A well woman taking a high-dose hair supplement has a very high free thyroxine with an undetectable thyroid stimulating hormone, but no symptoms and a normal pulse. Repeat testing after five days off the supplement is entirely normal. What happened?

Look first: set the results against the patient, then ask how a single substance could push two tests in opposite directions.

AThyroid hormone resistance
BTransient thyrotoxicosis
CA haemolysed first sample
DBiotin assay interference

Show the answer and explanation

Answer: D. Biotin assay interference

Two assay diagrams side by side, each anchored by a biotin and streptavidin link. In the sandwich assay, free biotin saturates the streptavidin, the complex is washed away and the result reads falsely low. In the competitive assay, the same loss of signal makes the result read falsely high. A results panel shows a very high free thyroxine with an undetectable stimulating hormone in a woman with a normal pulse, and a repeat panel five days after stopping the supplement is normal. — Recallab question 54614. Two assay formats, one interferent, and results that move opposite ways, from the question's explanation.

D. Biotin assay interference. Many immunoassays anchor the antibody complex with a biotin and streptavidin link, and a high circulating biotin concentration saturates those binding sites. In a competitive assay, as used for free thyroxine, less signal is captured and the result reads high. In a sandwich assay, as used for TSH, the complex is not captured and the result reads low. The pattern is physiologically impossible in a well patient, and it disappears once the supplement is stopped, because biotin is cleared quickly.

Why the other options lose: Thyroid hormone resistance (A) is genetic and permanent, with a normal or raised TSH. Transient thyrotoxicosis (B) of that severity would not resolve in five days, and she had no signs. A haemolysed first sample (C) neither raises thyroid hormones nor suppresses TSH.

Recallab question 54614 · Biochemistry › Clinical Biochemistry · reference: Harper's Illustrated Biochemistry

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When the binding globulin moves

Recallab example 6 · a stable dose that stops working

A woman of 52 had a total thyroidectomy eight years ago and has been stable on the same levothyroxine dose since. She started hormone replacement for hot flushes at month 0. Her results are drawn in the figure. What explains the change? A four-column results table running from two months before to four months after.

Look first: find the month the results start to drift, then read the row of other tablets for that month.

A results table with four columns headed Month minus 2, Month 0, Month 2 and Month 4. TSH reads 1.8, 2.0, 4.9 and 6.2 milliunits per litre. Free T4 reads 17, 16, 13 and 11 picomoles per litre. Levothyroxine reads 100 micrograms in all four columns. Other tablets reads none, none, then oral estradiol with a progestogen at months 2 and 4. — Recallab question 60576. TSH and free T4 before and after hormone replacement, on an unchanged levothyroxine dose.

ASuppression of her remaining thyroid tissue
BAssay interference by oestrogen
CProgestogen binding levothyroxine in the gut
DRaised thyroxine-binding globulin

Show the answer and explanation

Answer: D. Raised thyroxine-binding globulin

D. Raised thyroxine-binding globulin. The table is stable until month 0 and the dose never changes. Oral oestrogen passes through the liver in high concentration, and the liver makes more thyroxine-binding globulin. More hormone is bound, so the free fraction falls. A normal gland simply makes more, and its TSH hardly moves; after thyroidectomy there is no gland to compensate, so the TSH climbs. The levothyroxine dose is raised, or transdermal oestradiol is used instead, because it skips the first pass and leaves the globulin unchanged.

Why the other options lose: Suppression of her remaining thyroid tissue (A) is impossible after total thyroidectomy. Assay interference by oestrogen (B) is wrong because the change is real, tracks the start of treatment and matches a rising TSH. Progestogen binding levothyroxine in the gut (C) does not happen; calcium and iron are the tablets to separate from levothyroxine.

Recallab question 60576 · Gynaecology › Menstrual & Endocrine · reference: Williams Gynecology

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The one-minute routine

If the question gives	Ask first	It settles
A low free T4 with a TSH that is not high	Can the pituitary respond?	Central hypothyroidism: check cortisol before thyroxine
A suppressed TSH in early pregnancy	Is the pregnancy hormone at its peak?	Gestational transient thyrotoxicosis: no antithyroid drug
A raised TSH while recovering from severe illness	What was the TSH before and during the illness?	A recovery overshoot: repeat the tests later
A high T4 and T3 with a TSH that is not suppressed	Is there a pituitary tumour, or an affected parent?	Resistance is left alone; an adenoma is removed
Impossible results in a well patient	Is she taking a supplement?	Biotin interference: stop it and repeat the tests
A stable dose that stops working	Has a new tablet changed the binding globulin?	Oral oestrogen raises it: increase the dose or change the route

Practise endocrinology questions and obstetrics questions. For another set of results that are read in pairs, see acid-base and electrolytes.

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Drawn from questions #55854, #53995, #74240, #70003, #54614, #60576 in the Recallab bank, each with its own verified answer and current-edition reference. Last reviewed 27 October 2026.