Thyroid Hormones: TSH, T4 & T3

Thyroid hormones set the pace of metabolism throughout the body. The pituitary signal TSH, together with the thyroid's own hormones T4 and T3, forms a tightly controlled system that keeps energy use in balance. Understanding how these three messengers relate to one another makes thyroid blood tests far easier to read.

What thyroid hormones are

The thyroid gland produces two main hormones: thyroxine (T4) and triiodothyronine (T3). The numbers refer to how many iodine atoms each molecule carries — four for T4 and three for T3. T4 is the more abundant form and acts largely as a circulating reservoir, while T3 is the more biologically active form that actually engages with cells. Both are built around the amino acid tyrosine combined with iodine, which the body obtains from the diet.

Thyroid-stimulating hormone (TSH) is not made by the thyroid at all. It comes from the pituitary gland and is the instruction that tells the thyroid how hard to work. Because TSH and the thyroid hormones it controls move in a predictable relationship, clinicians can often infer a great deal about thyroid function from these few measurements together.

Where they are produced

T4 and T3 are made in the thyroid, a butterfly-shaped gland in the front of the neck wrapped around the windpipe. The gland traps iodine from the bloodstream and assembles it onto a large protein called thyroglobulin, from which the finished hormones are released. The great majority of what the thyroid secretes is T4. Much of the active T3 in the body is actually produced outside the thyroid, when other tissues — especially the liver and kidneys — convert T4 into T3 as needed. TSH is produced by the anterior pituitary gland at the base of the brain, under the direction of the hypothalamus.

How T4 becomes T3

Because T4 is largely a storage and transport form, the body converts it into the more active T3 through enzymes called deiodinases, which remove one iodine atom. This conversion lets each tissue fine-tune how much active hormone it receives. Some T4 is instead converted to an inactive molecule called reverse T3, which provides a way to dial activity down — for example, during serious illness or starvation. This local control means that the level of hormone acting inside a given tissue is not always identical to the level circulating in the blood.

Free versus bound hormone

Most thyroid hormone in the blood travels attached to carrier proteins, chiefly thyroxine-binding globulin. Only a small fraction circulates unattached, and it is this free fraction that is available to enter cells and act. This is why laboratories often report free T4 and free T3 rather than total levels: the free measurement is less affected by conditions that change carrier-protein amounts. Pregnancy, estrogen-containing medicines, and some liver conditions can raise binding proteins and shift total readings without necessarily changing the active free hormone, which is one reason results are interpreted as a pattern rather than a single number.

What they do in the body

Thyroid hormones act on nearly every tissue, which is why their effects are so wide-ranging.

• Metabolism and energy: They influence how quickly cells use energy and generate heat, affecting weight, body temperature, and energy levels.
• Heart and circulation: They affect heart rate and how forcefully the heart beats, as well as blood vessel tone.
• Brain and nervous system: They are essential for brain development in infancy and support mood, alertness, and concentration throughout life.
• Growth and development: In children, adequate thyroid hormone is necessary for normal growth and bone maturation.
• Digestion and other systems: They influence bowel motility, skin, hair, and menstrual patterns.

How levels are regulated

Thyroid hormones are governed by the hypothalamic–pituitary–thyroid (HPT) axis. The hypothalamus releases thyrotropin-releasing hormone (TRH), which prompts the pituitary to release TSH. TSH then stimulates the thyroid to produce T4 and T3. As T4 and T3 rise, they feed back to the hypothalamus and pituitary to reduce TRH and TSH — a negative feedback loop. Because of this loop, TSH often moves in the opposite direction to thyroid hormone: when thyroid hormone is low, TSH tends to rise to push the thyroid harder, and when thyroid hormone is high, TSH tends to fall.

What can raise or lower thyroid hormone

Several everyday and medical factors can shift the system. Iodine intake matters because iodine is a raw material; both too little and, in some people, sudden excess can disturb hormone production. Autoimmune processes can either damage the gland or overstimulate it. Pregnancy changes binding proteins and hormone demand, and the postpartum period can bring temporary swings. Serious non-thyroid illness can lower active hormone levels through reduced T4-to-T3 conversion, a pattern sometimes called non-thyroidal illness. A number of medicines, including some heart, mood-stabilising, and contrast agents, can also influence thyroid measurements.

What high or low levels can be associated with

An underactive thyroid (hypothyroidism) is often associated with fatigue, feeling cold, weight gain, and sluggish metabolism, and typically shows a higher TSH with lower thyroid hormone. An overactive thyroid (hyperthyroidism) is often associated with a faster heartbeat, weight loss, tremor, and feeling overheated, typically with a low or suppressed TSH and raised thyroid hormone. These are qualitative patterns; see the conditions index and consult a clinician.

The directions above are illustrative and simplified; real interpretation varies by laboratory, age, sex, pregnancy status, and the clinical picture.

How they are measured in blood

Thyroid testing usually starts with TSH because it is a sensitive early indicator — small changes in thyroid hormone produce relatively larger changes in TSH. Depending on the result, a clinician may add free T4 and sometimes free T3, and occasionally thyroid antibodies to look for an autoimmune cause. See the blood tests overview for context, and the TSH test guide for detail.

Measurement nuances

Several practical points shape how thyroid results are read. TSH can vary through the day and is generally most stable in the morning. Because the feedback loop takes time to settle, TSH may lag behind a recent change in thyroid hormone, so results soon after a change can be misleading. Different laboratories use different assays and report slightly different reference ranges, which is why comparing values across labs requires care. Pregnancy has its own expected ranges that differ from the general population. Some supplements and substances, including high-dose biotin, can interfere with certain assays. For all these reasons, a clinician interprets the trend across tests and the clinical picture rather than reacting to a single figure.

Established physiology versus open questions

The core picture — the HPT axis, the conversion of T4 to T3, and negative feedback — is well established. Other areas remain under study, such as how best to interpret mildly abnormal TSH with normal thyroid hormone in people without symptoms, and how individual differences in T4-to-T3 conversion affect how people feel. Where the evidence is still developing, conservative interpretation and clinical judgement matter most.