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.