Understanding Your Thyroid Blood Tests: TSH, T4, and T3 Explained

When your healthcare provider orders thyroid blood tests, they're checking three key hormones that work together to regulate your body's metabolism. Understanding what these hormones do can help you make sense of your test results.uclahealth+1​

The Three Main Thyroid Hormones

TSH (Thyroid-Stimulating Hormone)

TSH is produced by your pituitary gland, a small gland in your brain that acts like a thermostat for your thyroid. When your pituitary senses that thyroid hormone levels are too low, it produces more TSH to tell your thyroid gland to make more hormones. Conversely, when thyroid hormone levels are high enough, your pituitary turns down TSH production. The normal TSH range is typically 0.4 to 5.0 mIU/L, though this can vary slightly between laboratories.clevelandclinic+3​

T4 (Thyroxine)

T4 is the primary thyroid hormone circulating in your bloodstream, making up about 80-95% of the hormone your thyroid produces. Think of T4 as the storage form of thyroid hormone that your body keeps in reserve. Most T4 gets converted into T3 (the active form) mainly in your liver and other tissues where it's needed. The normal free T4 range for adults is approximately 0.7 to 1.9 ng/dL or 9 to 25 pmol/L.vinmec+3​

T3 (Triiodothyronine)

T3 is the active form of thyroid hormone that actually does the work in your body's cells. Only about 5-20% of the thyroid hormone in your blood is T3, with most of it created by converting T4 to T3 in your liver and other tissues. Normal T3 levels in adults typically range from 80-220 ng/dL or 3.5 to 7.8 nmol/L.geekymedics+3​

How These Hormones Work Together

Your body maintains thyroid hormone balance through a feedback loop system. When your brain detects low thyroid hormone levels, it releases thyroid-releasing hormone (TRH), which tells your pituitary to produce TSH. TSH then signals your thyroid to make more T4 and T3. When thyroid hormone levels rise high enough, this feedback loop shuts off TRH and TSH production, keeping everything in balance.clevelandclinic+1​

What Your Test Results Mean

Hypothyroidism (Underactive Thyroid)

When your thyroid doesn't produce enough hormone, TSH levels rise as your pituitary tries harder to stimulate the thyroid, while T4 and T3 levels drop. An elevated TSH is the most accurate indicator of hypothyroidism. Sometimes TSH rises even when T4 and T3 are still in the normal range, catching the problem early.aace+2​

Hyperthyroidism (Overactive Thyroid)

When your thyroid produces too much hormone, the excess T4 and T3 cause your pituitary to shut down TSH production through negative feedback. This results in high T4 and T3 levels with a low TSH. In some cases, only T3 is elevated while T4 remains normal, a condition called T3-toxicosis.thyroid+4​

Why Doctors Usually Test TSH First

TSH provides the frontline investigation of thyroid disease because it's extremely sensitive to changes in thyroid hormone levels. If your TSH is normal, your thyroid function is usually fine. If TSH is abnormal, your doctor will then check T4 and sometimes T3 levels to understand the full picture.ncbi.nlm.nih+2