What TSH, free T3, and free T4 actually measure

Thyroid function tests are among the most frequently requested panels at Cedar Pulse Lab, often ordered by patients who are investigating fatigue, unexplained weight changes, or changes in mood and concentration. The three-marker panel. TSH, free T3, and free T4. Gives a reasonably complete picture of how the thyroid gland is functioning, but the relationship between the markers is not always.

What each marker measures ¶

TSH, or thyroid-stimulating hormone, is produced by the pituitary gland and acts as a signal to the thyroid to produce more or less hormone. When thyroid hormone levels in the blood are low, the pituitary increases TSH output to stimulate the gland. When levels are high, TSH drops. This inverse relationship means that TSH is often the most sensitive early indicator of thyroid dysfunction: it changes before the thyroid hormones themselves move outside the reference range.

Free T4 (thyroxine) is the main hormone produced by the thyroid gland. Most of it circulates bound to proteins and is inactive; the free fraction is what the body can actually use. Free T3 (triiodothyronine) is the more biologically active form, produced partly by the thyroid and partly by conversion of T4 in peripheral tissues.

Common patterns and what they suggest ¶

A high TSH with a low or low-normal free T4 is the typical pattern of primary hypothyroidism, where the thyroid is underproducing hormone and the pituitary is compensating by increasing its signal. A low TSH with a high free T4 or free T3 suggests hyperthyroidism. Subclinical hypothyroidism. Elevated TSH with normal free T4. Is a common finding that may or may not require treatment depending on the degree of elevation and the patient's symptoms.

These patterns are a starting point for a conversation with your physician, not a diagnosis. The plain-language summary attached to every thyroid result at Cedar Pulse Lab notes which markers are flagged and what the pattern typically suggests, so you can arrive at your appointment with a clearer question rather than a page of numbers.

Fasting and timing ¶

The thyroid panel does not require fasting, and there is no strict requirement to collect it at a specific time of day for most patients. TSH does have a mild diurnal variation. It tends to be slightly higher in the early morning. But for routine screening this is not clinically significant. If you are monitoring an existing thyroid condition and your physician has asked you to test at a consistent time, note that on your booking.

If you have been tested before and want to compare results over time, bring your previous reports to your appointment. Trend data is often more informative than a single result in isolation.