TSH: What This Hormone Is and What Thyrotropin Does

Many people believe that TSH is a hormone of the thyroid gland itself, something like its report on how it is working. In fact, the gland does not produce it, and the logic here is the reverse. Below is what produces it and why it comes first on test requisitions.

Thyroid-stimulating hormone: what it is

Thyroid-stimulating hormone — also known as TSH, thyrotropin, thyrotropin, TSH — is a tropic hormone of the anterior pituitary gland. Structurally it is a glycoprotein made of two subunits, α and β, linked by a non-covalent bond. The α-subunit is also found in other hormones, while β provides specific binding to the receptor. Thyrotropin receptors are located on the surface of the epithelial cells of the thyroid gland.

Where TSH is produced and how it is connected to the thyroid

It is produced in the anterior pituitary gland, and its release is regulated by a negative feedback system and by releasing factors from the neurosecretory cells of the hypothalamus. The connection with the thyroid is mirror-like: there is an inverse relationship between the concentrations of free T4 and TSH in the blood. When T4 is higher, TSH production falls; when T4 is lower, it rises. So TSH tells you not about the gland directly, but about how loudly the pituitary is asking it to work.

What TSH does in the body

By acting on receptors in the gland, thyrotropin stimulates the production and activation of thyroxine: it activates adenylate cyclase, increases iodine uptake by cells, and the rise in cAMP triggers the biosynthesis of T3 and T4. There are also long-term effects that take days: more synthesis of proteins, nucleic acids, and phospholipids, and larger and more numerous thyroid cells. It also increases tissue sensitivity to thyroid hormones, preparing them in advance for their action. With prolonged exposure at high concentrations, it causes proliferation of the gland tissue, increasing its size and mass.

What a TSH blood level shows

TSH is usually the first test a doctor orders when a thyroid disease is suspected: it measures the level of thyroid-stimulating hormone. Then, depending on the situation, T3 and T4 and thyroid antibodies are checked; for imaging, ultrasound, CT, scintigraphy, and the radioactive iodine uptake test are used. A single number decides nothing: thyrotropin has daily fluctuations — a maximum at 2–4 a.m., slightly lower until 6–8 a.m., and a minimum at 5–7 p.m. Staying awake at night disrupts the rhythm, during pregnancy the concentration decreases, and with age TSH rises slightly.

When a TSH test is ordered

The reason is usually complaints that are easy to attribute to fatigue and age:

• marked fatigue and a feeling of being colder than usual;
• weight gain and constipation;
• difficulty concentrating and low mood;
• dry skin, dry hair or hair loss, a hoarse voice;
• irregular or heavy periods.

Symptoms develop slowly, and at first they are almost unnoticeable. The doctor will ask about them, about thyroid and autoimmune diseases in relatives, about a recent pregnancy, examine the neck — whether the gland is enlarged — and order blood tests for TSH and T4. If necessary, they will refer you to a specialist. Among the causes are autoimmune Hashimoto's disease (more common in women, usually beginning at 30–50 years of age), thyroiditis, surgery and radiotherapy in the neck area, iodine deficiency, and some medications.

When to see a doctor

If such complaints persist and interfere with life, you should see a doctor, even when everything is blamed on overwork: the symptoms are nonspecific and occur in other conditions. Pregnancy or plans for it are a separate matter: this should be reported to the doctor, because untreated hypothyroidism increases the risk of preeclampsia, birth defects, preterm birth, and miscarriage. Long-untreated hypothyroidism has delayed risks: heart problems, including coronary artery disease and heart failure, and bone problems, including osteoporosis.

An altered TSH by itself is not a reason to call an ambulance. 103 or 112 — if signs of myxedema coma appear in severe untreated hypothyroidism: this is a rare life-threatening condition, more often in older people, and requires hospital treatment.

This material is for reference and does not replace a doctor's consultation: it is better to go over the numbers on the form with someone who sees the whole picture.

This content is for reference and does not replace a doctor's consultation. Medical disclaimer