Thyroid Function Test Interpretation
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Direct answer
Read TSH first and free T4 second, and the four patterns solve themselves: high TSH with low free T4 is primary hypothyroidism; low TSH with high free T4 is primary thyrotoxicosis; low or inappropriately normal TSH with low free T4 points to central (pituitary) hypothyroidism; and both normal means euthyroid. The traps live at the edges — non-thyroidal illness, pregnancy, and drugs such as amiodarone, glucocorticoids, dopamine and biotin all distort the picture. Because TSH logarithmically amplifies free T4 changes, a normal TSH makes significant thyroid dysfunction very unlikely in an outpatient.
What you must remember
- Primary hypothyroidism: TSH up, free T4 down. Primary thyrotoxicosis: TSH suppressed (often <0.1 mIU/L), free T4 up.
- Central hypothyroidism: free T4 low with TSH low or "normal" — the inappropriately normal TSH is the clue; never diagnose it from TSH alone.
- Subclinical disease: TSH abnormal with normal free T4 — confirmed on repeat because TSH varies up to 20% day to day and peaks at night.
- Euthyroid sick (non-thyroidal illness) pattern: T3 low first, free T4 low or normal, reverse T3 raised, TSH low or modestly raised — do not treat, retest after recovery.
- Pregnancy: hCG stimulates the TSH receptor, so TSH falls in the first trimester; use trimester-specific ranges (first trimester roughly 0.1–2.5 mIU/L).
- TSH-suppressing drugs: glucocorticoids, dopamine/dobutamine, octreotide, bexarotene. T4-lowering drugs: phenytoin, carbamazepine, rifampicin (accelerated clearance).
- Biotin interference: megadose biotin supplements falsely raise free T4 and lower TSH in streptavidin-biotin immunoassays, perfectly mimicking Graves' disease — ask about hair and skin supplements.
- TSH is the screening test; add free T4 when TSH is abnormal, in suspected central disease, or during pregnancy monitoring.
Working through the odd patterns
Take a pattern-based walk. A 45-year-old with TSH 0.05 mIU/L and free T4 28 pmol/L (high) has primary thyrotoxicosis; add TRAb and a radioiodine uptake scan if the cause is unclear. Now keep the free T4 low — say 8 pmol/L — and the TSH reads 1.2 mIU/L, "normal". That pairing is physiologically impossible in a healthy axis: a truly low T4 should drive TSH above 20. This is central hypothyroidism until proved otherwise; look for a pituitary cause, check other axes, and image the sella.
Next, the intensive-care patient with TSH 0.2, free T4 9, T3 low. Starvation and critical illness shift the deiodinase economy: T3 falls, reverse T3 climbs, and everything normalises on recovery — replacing hormone does not help and testing during the acute phase is best avoided unless suspicion is high.
Then the biotin trap, increasingly relevant as supplement use grows in Indian metros. A young woman on 10 mg biotin daily for hair fall returns "thyrotoxic": TSH <0.01, free T4 high. Clinically she is calm, with no tachycardia or tremor, and her free T3 is oddly normal. Stop biotin for 48–72 hours (at least two days, longer for high doses) and repeat — the pattern dissolves. Finally, pregnancy: a first-trimester TSH of 0.15 with high-normal free T4 is usually physiological hCG action, not Graves, though a past history of Graves or antithyroid drugs warrants TRAb testing.
Where students slip
Two errors dominate. The first is calling a low TSH with low free T4 "secondary hyperthyroidism" — a non-existent entity in standard teaching; central disease lowers free T4, full stop. The second is over-reading single abnormalities: a mildly raised TSH during an acute illness, after steroid exposure, or at night (TSH peaks around midnight) means little until repeated under basal conditions. A less common but examinable slip is forgetting that in central hypothyroidism TSH can be mildly elevated from biologically inactive immunoreactive TSH — free T4, not TSH, tracks the disease.
Frequently asked questions
Which single test is the best initial screen for thyroid dysfunction?
TSH, in an ambulatory patient, because its logarithmic relationship to T4 makes it the most sensitive marker of primary dysfunction. Add free T4 when TSH is abnormal, when central hypothyroidism is suspected, or in pregnancy.
What pattern characterises euthyroid sick syndrome?
Low T3 with low or normal free T4, raised reverse T3, and variable TSH. Treatment is of the underlying illness, with repeat testing after recovery rather than levothyroxine.
How does pregnancy change thyroid test interpretation?
hCG cross-stimulates the TSH receptor, lowering TSH — particularly by 8–10 weeks — so first-trimester TSH readings must be judged against trimester-specific ranges. Total T4 rises with thyroxine-binding globulin, so free T4 (ideally by the right assay method) is preferred.
Why can biotin mimic Graves' disease on testing?
Many immunoassays use biotin-streptavidin capture; circulating megadose biotin interferes with the signal, falsely elevating free T4/T3 and suppressing measured TSH. The patient is clinically euthyroid, and retesting after stopping biotin normalises results.
When should central hypothyroidism be suspected from bloods?
When free T4 is low but TSH is low or inappropriately normal. Confirm with other pituitary axes and MRI of the pituitary, and remember to cover with glucocorticoids before starting levothyroxine if adrenal insufficiency coexists.