How Do You Read Thyroid Tests, From TSH to Free T4 and T3?

The first thyroid test is usually thyroid-stimulating hormone (TSH), and it works backwards: TSH is the pituitary gland’s signal to the thyroid, so when the thyroid makes too little hormone, TSH rises, and when it makes too much, TSH falls. Read together with free thyroxine (free T4), the main thyroid hormone, TSH sorts most thyroid problems into a handful of patterns.

This article is general information, not medical advice. Thyroid results are best interpreted by a clinician, alongside your symptoms and medicines.

The Tests

• TSH, from the pituitary. The most sensitive test of thyroid function, because small changes in thyroid hormone cause large changes in TSH.
• Free T4 (free thyroxine), the unbound, active share of the thyroid’s main hormone.
• Free T3 and total T3 (triiodothyronine), the more active hormone, mostly made from T4 in other tissues. Most useful when an overactive thyroid is suspected.
• TPO antibodies (thyroid peroxidase antibodies), a sign of autoimmune thyroid disease such as Hashimoto’s thyroiditis.

The Patterns

TSH	Free T4	Usually means
High	Low	Hypothyroidism (underactive thyroid)
High	Normal	Subclinical hypothyroidism
Low	High	Hyperthyroidism (overactive thyroid)
Low	Normal	Subclinical hyperthyroidism (check T3 too)
Normal	Normal	Normal thyroid function

The 2012 hypothyroidism guidelines of the American Association of Clinical Endocrinologists and the American Thyroid Association (Garber and colleagues) recommend TSH as the primary test, and note that TPO antibodies predict a higher chance of progressing from subclinical to overt hypothyroidism. Whether to treat a mildly raised TSH with a normal free T4 depends on its level, symptoms, age, and pregnancy plans.

What Throws Results Off

• Biotin supplements, common in hair, skin, and nail products and some multivitamins, can interfere with hormone tests that use biotin in their chemistry, the US Food and Drug Administration (FDA) warns, including some thyroid tests. Tell your clinician and lab if you take it.
• Acute illness temporarily changes thyroid tests without thyroid disease.
• Pregnancy changes the normal ranges.
• Thyroid medicine timing. If you take levothyroxine, follow your clinician’s advice about testing before or after your dose.
• Time of day. TSH is higher at night and early morning, so draw at a consistent time.

Each lab has its own reference ranges, which differ by method.

Following It in Kaizora

Thyroid-Stimulating Hormone, Free Thyroxine, Free Triiodothyronine, Total Triiodothyronine, and Thyroid Peroxidase Antibodies are lab results on Labs, each logged with its time, laboratory, and printed range, so a result outside the lab’s range is flagged. A draw’s results share their time and laboratory, so the TSH and free T4 from the same blood line up. Since TSH changes slowly with treatment, its chart over months shows a dose adjustment’s effect.

The Short Version

• TSH rises when the thyroid is underactive and falls when it’s overactive.
• TSH with free T4 identifies most thyroid problems; TPO antibodies point to autoimmune disease.
• Biotin, illness, pregnancy, and time of day can all affect the results.

Sources

• Garber JR et al., “Clinical practice guidelines for hypothyroidism in adults: cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association,” Thyroid 22, no. 12 (2012): 1200–1235. doi:10.1089/thy.2012.0205
• US Food and Drug Administration, Testing for Biotin Interference in In Vitro Diagnostic Devices (guidance).