Step by Step
1
TSH — the most sensitive screening test
TSH (0.4-4.0 mU/L) reflects how hard the pituitary is working to stimulate the thyroid. High TSH means the pituitary is working hard to stimulate a failing thyroid — hypothyroidism. Low TSH means the pituitary has been suppressed by excess thyroid hormone — hyperthyroidism.
2
Free T4 — confirms and grades severity
Free T4 (0.8-1.8 ng/dL) represents the unbound, biologically active form of thyroid hormone, used to confirm and grade the severity of a thyroid abnormality once TSH has flagged a potential problem.
3
Primary thyroid disorders — the classic patterns
Primary hypothyroidism shows increased TSH plus decreased T4, treated with levothyroxine. Primary hyperthyroidism shows decreased TSH plus increased T4, most commonly caused by Graves' disease.
4
Less common patterns worth recognizing
Secondary hypothyroidism (from pituitary failure) shows decreased TSH plus decreased T4 — since the pituitary itself isn't producing enough TSH to stimulate the thyroid in the first place. Subclinical hypothyroidism shows increased TSH with a normal T4 — typically only treated if TSH exceeds 10 or the patient is symptomatic.
Applied Walkthrough
1
A patient with fatigue and weight gain has an elevated TSH — the pituitary working overtime to stimulate an underperforming thyroid — and a follow-up free T4 confirms it's low, establishing a diagnosis of primary hypothyroidism, typically treated with levothyroxine.
2
A different patient with weight loss and palpitations has a suppressed TSH, with follow-up free T4 confirming it's elevated — primary hyperthyroidism, most commonly from Graves' disease.
3
A third patient's labs are unusual: both TSH and T4 are low. This pattern doesn't fit either of the classic primary disorders — instead, it suggests secondary hypothyroidism, where the pituitary itself has failed and isn't producing enough TSH to properly stimulate an otherwise normal thyroid.
4
A fourth, asymptomatic patient has mildly elevated TSH but a normal T4 — subclinical hypothyroidism, which would typically only be treated if the TSH climbs above 10 or symptoms develop.
Exam Application
Exams test whether you understand why TSH is the primary screening test (reflecting pituitary feedback), and whether you can correctly match TSH/T4 patterns to primary hypothyroidism, primary hyperthyroidism, secondary hypothyroidism, and subclinical hypothyroidism.
⚠ Common Trap
The most common trap is assuming low TSH and low T4 together always means hypothyroidism is ruled out — this specific pattern actually indicates secondary hypothyroidism (pituitary failure), a distinct and less common diagnosis from the classic primary hypothyroidism pattern (high TSH, low T4).
✓ Quick Self-Check
1. Why is TSH considered the best screening test for thyroid function?
Because it's the most sensitive test, reflecting how hard the pituitary is working to stimulate the thyroid via feedback.
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2. What TSH/T4 pattern indicates primary hypothyroidism?
High TSH, low T4.
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3. What TSH/T4 pattern indicates primary hyperthyroidism, and what is its most common cause?
Low TSH, high T4; most commonly caused by Graves' disease.
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4. What TSH/T4 pattern indicates secondary hypothyroidism, and why?
Low TSH, low T4 — because the pituitary itself has failed and isn't producing enough TSH to stimulate the thyroid.
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5. What defines subclinical hypothyroidism, and when is it typically treated?
High TSH with normal T4; typically treated if TSH exceeds 10 or the patient is symptomatic.
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