The thyroid gland is a small butterfly-shaped organ on the front of the neck. It produces the hormones T4 (thyroxine) and T3 (triiodothyronine), which set the metabolic rate in literally every cell of the body.
That is why its malfunctions produce such a varied picture: energy, weight, body temperature, the heart, the gut, the skin, hair, mood and memory all suffer. The gland is controlled by the pituitary via TSH on a feedback principle.
Reference ranges
The basic panel to assess function:
| TSH | 0.4 – 4.0 mIU/L (optimum 0.5 – 2.5) |
| Free T4 | 0.8 – 1.8 ng/dL |
| Free T3 | 2.3 – 6.2 pg/mL |
| TPO antibodies | below 34 IU/mL |
What an elevated level means
Overactivity (hyperthyroidism) — too much hormone, metabolism revved up. TSH is reduced, free T4/T3 elevated.
Symptoms: palpitations, hand tremor, sweating, heat intolerance, anxiety, insomnia, weight loss with a good appetite, frequent stools.
Causes: Graves' disease (autoimmune), toxic nodular goiter, thyroiditis in the hormone-release phase, levothyroxine overdose. Untreated hyperthyroidism raises the risk of atrial fibrillation and osteoporosis.
What a low level means
Underactivity (hypothyroidism) — not enough hormone, metabolism slowed. TSH is elevated, free T4 reduced (or still normal — then it is subclinical hypothyroidism).
Symptoms: fatigue, feeling cold, weight gain, dry skin, hair loss, constipation, "brain fog", low mood, puffiness, a slower pulse.
The main cause is autoimmune thyroiditis (Hashimoto's), so TPO antibodies are looked at along with TSH. Also: iodine deficiency, a state after surgery or radioiodine therapy.
How the test is done
Start with TSH. Blood is drawn in the morning, under consistent conditions: TSH has a daily rhythm (higher in the morning). Strict fasting is not required.
Stop biotin (vitamin B7) 2–3 days beforehand — in high doses it seriously distorts thyroid test results. Do not test during an acute illness: it can give a false picture (euthyroid sick syndrome).
What affects the value
- Iodine — both deficiency and excess disrupt the gland.
- Selenium and zinc — needed to convert T4 into active T3.
- Autoimmune processes — Hashimoto's thyroiditis is the most common cause of hypothyroidism.
- Biotin — distorts test results (not the gland itself).
- Stress and chronic illness — reduce peripheral conversion of T4 to T3.
- Medications — amiodarone, lithium, interferons, glucocorticoids.
Calculate using this marker
Calculators where Thyroid gland is used directly:
Frequently asked questions
Where do I start checking the thyroid?
With a TSH test — it is the most sensitive and changes earliest. If there are complaints (fatigue, weight, feeling cold), add free T4 and TPO antibodies right away: TSH alone may not reveal subclinical hypothyroidism.
TSH is normal but I have symptoms — what next?
A normal TSH does not guarantee there is enough active T3 in the tissues. This happens with impaired peripheral conversion of T4 to T3. Then free T3 and the free-T3/free-T4 ratio are looked at, not just TSH.
Does everyone need a thyroid ultrasound?
No. Ultrasound shows structure (nodules, volume) but says nothing about function. Function screening is a blood test. Ultrasound is done for palpable nodules, neck changes, or as ordered by an endocrinologist.