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Printed on 6/29/2026
For informational purposes only. This is not medical advice.
This tool interprets thyroid-stimulating hormone (TSH) and optional free T4 levels to provide a clinical assessment of thyroid function. TSH is the most sensitive initial screening test for thyroid disorders. Combining TSH with free T4 allows differentiation between overt and subclinical conditions, primary and central disorders, and guides further workup. Uncontrolled hyperthyroidism increases atrial fibrillation risk — assess stroke risk with [CHA2DS2-VASc Score](/tools/cha2ds2-vasc) and consider anticoagulation (weigh bleeding risk with [HAS-BLED Score](/tools/has-bled)). Hypothyroidism contributes to weight gain — track with [BMI Calculator](/tools/bmi-calculator).
Formula: Clinical interpretation based on TSH and free T4 reference ranges
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Input your TSH result from the lab report. The normal reference range is approximately 0.4–4.5 mIU/L. Third-generation TSH assays can detect values as low as 0.02 mIU/L, enabling differentiation of suppressed from undetectable TSH.
Enter your free T4 if available (normal approximately 0.8–1.8 ng/dL). Combining TSH with free T4 allows the tool to distinguish overt from subclinical thyroid disease and identify the rare pattern of central hypothyroidism.
The tool categorizes your results as euthyroid, overt or subclinical hypothyroidism, overt or subclinical hyperthyroidism, or a discordant pattern needing further workup. Clinical context — symptoms, medications, pregnancy — is always needed alongside lab values.
Primary care physicians, internists
Screen patients with risk factors for thyroid disease: family history, autoimmune conditions, prior neck radiation, or age over 60. TSH alone is the recommended first-line test; free T4 is added only if TSH is abnormal.
Endocrinologists, PCPs, pharmacists
Assess whether a patient's levothyroxine dose is achieving the TSH target (typically 0.5–2.5 mIU/L for primary hypothyroidism). TSH should be checked 6–8 weeks after any dose adjustment, as it takes this long to reach a new steady state.
Endocrinologists, nuclear medicine teams
Track thyroid function recovery or ablation adequacy after RAI for hyperthyroidism or thyroid cancer. TSH and free T4 together indicate whether replacement therapy is needed and whether TSH suppression targets are being met.
Obstetricians, maternal-fetal medicine specialists
Screen for thyroid dysfunction in early pregnancy. hCG suppresses TSH physiologically in the first trimester, so pregnancy-specific ranges apply (typically TSH <2.5 mIU/L in first trimester per ATA 2017 Pregnancy Guidelines).
Patients, generalists
Evaluate symptoms that may have a thyroid cause: fatigue, weight change, temperature intolerance, palpitations, hair thinning, constipation, or mood changes. A single TSH result is the most efficient first test when thyroid disease is in the differential.
Geriatricians, hospitalists
Elderly patients often have atypical presentations and may tolerate higher TSH targets (4–6 mIU/L). Subclinical hyperthyroidism in older adults carries particular risk of atrial fibrillation and osteoporosis and warrants lower treatment thresholds.
Biotin (vitamin B7) — common in hair/nail supplements — interferes with immunometric assays and can produce falsely suppressed TSH or falsely elevated/decreased free T4, mimicking hyperthyroidism. Even low-dose biotin (1–5 mg) can cause clinically significant interference. Always ask about supplements before interpreting results.
TSH is highest around midnight and lowest in the mid-afternoon. Most clinical lab draws occur in the morning, which is the standard. If TSH is drawn at an unusual time, levels may be slightly different from morning reference ranges, though clinical significance is rarely major.
Human chorionic gonadotropin (hCG) shares structural similarity with TSH and stimulates the thyroid in the first trimester, physiologically suppressing TSH. The ATA 2017 Pregnancy Guidelines recommend trimester-specific TSH ranges; a TSH of 0.1–2.5 mIU/L in the first trimester is often normal.
Acute illness can suppress TSH and reduce free T4 without true thyroid disease — this is called non-thyroidal illness syndrome or sick euthyroid syndrome. Thyroid testing during hospitalization for acute illness is often misleading. Repeat testing after recovery is recommended.
In patients over 80 or frail older adults, an 'elevated' TSH of 4–6 mIU/L may be associated with longevity rather than harm. Aggressive treatment of mild subclinical hypothyroidism in this population can cause overtreatment and iatrogenic hyperthyroidism. Individualize goals with the patient.
In subclinical hypothyroidism (elevated TSH, normal free T4), the presence of thyroid peroxidase (TPO) antibodies indicates Hashimoto's thyroiditis and a 4–5% annual risk of progression to overt hypothyroidism, compared to about 2% without antibodies. A positive TPO antibody strengthens the case for closer monitoring or treatment.
In suppressed TSH with elevated free T4, thyrotropin receptor antibodies (TRAb or TSI) confirm Graves' disease with >95% specificity. This differentiates Graves' from toxic multinodular goiter or toxic adenoma, which are also TSH-suppressed but TRAb-negative and require different management.
Free T3 adds little to initial screening or monitoring but is useful in suspected T3 toxicosis (suppressed TSH, normal free T4, elevated free T3 — seen in some Graves' patients early in disease) and in amiodarone-induced thyroid disorders where T3 metabolism is altered.
In central (secondary) hypothyroidism due to pituitary or hypothalamic disease, TSH can be low, normal, or even slightly elevated despite low thyroid hormone. Always check free T4 in patients with known pituitary disease or when TSH and symptoms are discordant.
Different laboratories use different TSH assays (second vs. third generation) with different reference ranges. A TSH of 4.2 may be 'within normal limits' on one report and 'mildly elevated' on another. Always compare to the reference range printed on the patient's own lab report, not a generic value.
TSH measurement uses third-generation immunometric assays with functional sensitivity <0.02 mIU/L. Normal reference range was established by the NHANES III study. Subclinical disease definitions follow guidelines by the American Thyroid Association (ATA) and European Thyroid Association (ETA 2023). TSH in pregnancy follows ATA 2017 Pregnancy Guidelines.
Your results are interpreted based on where your TSH and free T4 values fall relative to standard reference ranges. A normal TSH (approximately 0.4-4.5 mIU/L) with a normal free T4 (approximately 0.8-1.8 ng/dL) indicates euthyroid status, meaning your thyroid is functioning normally. An elevated TSH with low free T4 suggests primary hypothyroidism, while a suppressed TSH with elevated free T4 suggests primary hyperthyroidism.
Subclinical conditions occur when TSH is abnormal but free T4 remains in the normal range. Subclinical hypothyroidism (elevated TSH, normal free T4) is common and may progress to overt hypothyroidism at a rate of approximately 2-5% per year, particularly if thyroid antibodies are present. Subclinical hyperthyroidism (low TSH, normal free T4) warrants monitoring and may require treatment if TSH remains persistently suppressed, especially in patients over 65 due to atrial fibrillation risk.
Rare patterns such as low TSH with low free T4 may suggest central hypothyroidism (a pituitary or hypothalamic problem) and require further endocrine evaluation.
Use this interpreter when reviewing thyroid function test results to understand the clinical significance of TSH and free T4 values. It is appropriate as a first step for patients presenting with symptoms suggestive of thyroid dysfunction such as fatigue, weight changes, temperature intolerance, palpitations, hair loss, or menstrual irregularities.
It is also useful for monitoring patients already on thyroid hormone replacement therapy (levothyroxine) to assess whether their current dose is achieving target TSH levels. In the monitoring setting, TSH should be checked 6-8 weeks after any dose change, as it takes this long for TSH to reach a new steady state.
Reference ranges for TSH vary between laboratories and assay methods. The ranges used in this tool are approximate and may not match your specific laboratory reference intervals. Always compare results to the reference range provided on your actual lab report.
TSH interpretation is affected by numerous confounders. Biotin supplements can interfere with immunoassays and produce falsely abnormal results, so patients should stop biotin at least 2 days before testing. Non-thyroidal illness (sick euthyroid syndrome) can suppress TSH and free T4 during acute illness without true thyroid disease. Pregnancy alters TSH reference ranges, with lower values expected in the first trimester due to hCG stimulation.
This tool provides pattern-based interpretation and does not replace clinical judgment. Discordant results (e.g., both TSH and free T4 elevated) may indicate assay interference, TSH-secreting pituitary adenoma, or thyroid hormone resistance, all of which require specialist evaluation.
For related assessments, see HbA1c Converter and Insulin Correction.
Disclaimer: This tool is for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider with questions about your health.
April 21, 2026 · trust-baseline
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