[Blog](https://vittalkamath.com/blog) # Thyroid hormone: thyroxine (T4) 15 May 2026 ## What it is T4 is the main hormone of the thyroid gland. It is a prohormone, which tissues convert into the active T3 (triiodothyronine). The thyroid itself makes about 80% T4 and 20% T3. ## Where it is made The thyroid is a butterfly-shaped gland at the front of the neck, under the skin. It needs iodine to make T4 and T3. ## The control system: the HPT axis A feedback loop runs it: 1. **Hypothalamus** → releases TRH (thyrotropin-releasing hormone). 2. **Pituitary gland** → releases TSH (thyroid-stimulating hormone). 3. **Thyroid** → makes T4 and T3. 4. High T4 and T3 shut off TRH and TSH. Low T4 and T3 turn them back on. ## How T4 works T4 itself is mostly inactive. The liver, kidneys, muscles, brain, pituitary and thyroid convert it to T3 by removing one iodine atom. T3 is what binds to cells and does the work. ## Main functions - Sets the metabolic rate, and how cells use calories. - Increases or decreases the heart rate. - Raises or lowers body temperature. - Controls how fast food moves through the gut. - Critical for brain development. - Regulates muscle contraction. - Maintains skin and bone. ## Key blood tests - **TSH:** the best screening test. Usually, high means hypothyroid and low means hyperthyroid. - **Free T4, free T3:** measure the unbound, active hormone. - **Total T4, total T3:** include the bound hormone; less useful on their own. - **Thyroid antibodies:** TPOAb for Hashimoto’s, TRAb for Graves’. - **Thyroglobulin:** a tumour marker used to monitor thyroid cancer. ## Common conditions - **Hypothyroidism:** too little hormone. Often from Hashimoto’s disease, an autoimmune attack on the gland. - **Hyperthyroidism:** too much hormone. Often from Graves’ disease, an autoimmune stimulation of the gland. - **Thyroiditis:** inflammation of the gland. - **Goitre:** an enlarged thyroid. - **Thyroid nodules:** lumps, most of them benign. - **Thyroid cancer:** less common, and usually treatable. ## See a doctor if you have Unexpected weight gain or loss, a fast or slow heart rate, intolerance of heat or cold, very dry or moist skin, or irregular periods. Thyroid conditions are highly treatable with medication, radioactive iodine or surgery, depending on the cause. For symptoms or treatment questions, talk to a healthcare professional. [All articles](https://vittalkamath.com/blog)