Which tests are used to diagnose thyroid function?

• A. Calcium and phosphate
• B. TSH, T3, and T4
• C. TSH only
• D. T3 and T4

Answer: (B)

The main concept is that assessing thyroid function requires both the regulatory signal from the pituitary and the actual thyroid hormones in the blood. TSH is the most sensitive first indicator: when thyroid hormone is low, TSH rises; when hormone is high, TSH falls. But to know exactly how the thyroid is performing, you also need the circulating hormones themselves—T4 (and its active form, free T4) and T3 (free T3 when needed). Putting these together gives a full picture: it shows whether the gland is underactive, overactive, or in a subclinical state where TSH is abnormal but hormone levels appear normal. Calcium and phosphate levels don’t tell you about thyroid function, so they aren’t used for this purpose. Relying on TSH alone can miss the true hormone status, and measuring T3/T4 without context from TSH can miss whether the axis is properly regulated. Therefore, testing both TSH and the thyroid hormones (T3 and T4) provides the most informative assessment of thyroid function.