TSH, T3, T4 Explained: How to Read Your Thyroid Function Test

Thyroid function tests are one of the most commonly ordered blood tests in India - and one of the most commonly misread. The source of confusion is almost always the same: TSH moves in the opposite direction to what you'd expect.

Once you understand why, the rest makes sense.

The thyroid system in 90 seconds

Your thyroid gland (a butterfly-shaped gland at the base of your neck) produces two hormones: T3 (triiodothyronine) and T4 (thyroxine). These hormones regulate your metabolism, body temperature, heart rate, weight, mood, and energy levels.

But your thyroid doesn't decide on its own how much to produce. It takes orders from your pituitary gland - a pea-sized gland at the base of your brain - via a hormone called TSH (Thyroid Stimulating Hormone).

Think of it like a thermostat:

• When T3/T4 levels drop, the pituitary senses this and pumps out more TSH to tell the thyroid to work harder.
• When T3/T4 levels are high, the pituitary cuts back TSH - "you're already producing enough, slow down."

This is why high TSH = underactive thyroid (hypothyroidism) and low TSH = overactive thyroid (hyperthyroidism) - the opposite of what instinct suggests.

Breaking down each test

TSH - Thyroid Stimulating Hormone

Normal range: 0.4–4.0 mIU/L (varies slightly by lab; some use 0.5–5.0)

TSH is the most sensitive indicator of thyroid status. For routine screening, most doctors order TSH alone. It changes before T3/T4 levels visibly shift - making it an early warning system.

| TSH Level | What It Suggests | |-----------|-----------------| | Below 0.4 | Possible hyperthyroidism (overactive) | | 0.4–4.0 | Normal | | 4.0–10.0 | Subclinical hypothyroidism | | Above 10.0 | Overt hypothyroidism (underactive) |

Subclinical hypothyroidism (TSH elevated, T4 normal) is extremely common in India - especially in women. It often has no symptoms but can affect fertility, mood, and cholesterol levels. Whether to treat it is a nuanced decision your doctor will make based on your specific situation.

Free T4 (FT4) - Free Thyroxine

Normal range: 0.8–1.8 ng/dL (or 12–22 pmol/L in SI units)

T4 is the main hormone the thyroid produces, but most of it is bound to carrier proteins in the blood and unavailable for use. Free T4 is the fraction that's actually active.

If TSH is abnormal, FT4 confirms the diagnosis:

• High TSH + Low FT4 = Overt hypothyroidism
• High TSH + Normal FT4 = Subclinical hypothyroidism
• Low TSH + High FT4 = Overt hyperthyroidism

Free T3 (FT3) - Free Triiodothyronine

Normal range: 2.3–4.2 pg/mL (or 3.5–6.5 pmol/L)

T3 is the active form that your cells actually use. Most T3 in the blood is converted from T4 in peripheral tissues (liver, kidneys), not secreted directly by the thyroid.

FT3 is generally ordered when:

• Hyperthyroidism is suspected but T4 is normal (T3-toxicosis is a real condition)
• Monitoring treatment for hyperthyroidism
• Assessing conversion issues (some people convert T4 to T3 poorly, even on thyroid medication)

For routine screening, FT3 alone adds little beyond TSH + FT4.

Common thyroid conditions explained

Hypothyroidism (underactive thyroid)

The thyroid isn't making enough hormone. TSH is high (pituitary is trying to stimulate it harder), FT4 is low.

Symptoms: Fatigue, weight gain, cold intolerance, hair loss, constipation, brain fog, low mood, heavy periods in women.

Most common cause in India: Hashimoto's thyroiditis - an autoimmune condition where the immune system attacks the thyroid. If suspected, your doctor will order Anti-TPO antibodies.

Treatment: Levothyroxine (synthetic T4), a daily pill, usually lifelong.

Hyperthyroidism (overactive thyroid)

The thyroid is producing too much hormone. TSH is suppressed (the pituitary is trying to dial it back), FT4 and/or FT3 is high.

Symptoms: Weight loss despite good appetite, rapid heartbeat, heat intolerance, tremors, anxiety, frequent bowel movements, irregular periods.

Most common causes: Graves' disease (autoimmune), toxic multinodular goitre, thyroiditis.

Treatment: Anti-thyroid drugs (carbimazole, propylthiouracil), radioactive iodine, or surgery - depending on the cause and severity.

Subclinical thyroid disease

TSH is outside normal range, but T4 and T3 are normal. Often no obvious symptoms. Very common in women, especially perimenopausal.

Whether to treat depends on:

• Degree of TSH deviation
• Symptoms
• Presence of antibodies (Anti-TPO)
• Plans for pregnancy
• Age and cardiovascular risk

Don't panic about a mildly elevated TSH - discuss with your doctor.

What affects your thyroid test results

• Time of day: TSH peaks at night and is lowest around noon. Morning tests give higher TSH values.
• Illness: Acute illness (even a bad cold) can temporarily suppress TSH.
• Medications: Biotin supplements (very commonly taken in India for hair loss) can falsely alter thyroid test results. Stop biotin 48 hours before your thyroid test.
• Pregnancy: Normal TSH ranges are lower during pregnancy. A value "normal" for a non-pregnant adult may be abnormal during pregnancy.
• Recent iodine exposure: Contrast dye for CT scans can affect thyroid function for weeks.

Questions to ask your doctor

1. My TSH is slightly elevated - given my symptoms and Anti-TPO status, should I start medication or watch and wait?
2. Am I on the right dose of levothyroxine, and should I retest my TSH in 6–8 weeks?
3. I'm taking biotin - did I need to stop before this test?
4. Should I also test Anti-TPO antibodies to understand the cause?
5. If I'm planning pregnancy, what TSH level should I aim for before conceiving?

ReportSense provides educational health information only - not medical diagnosis or advice. Always consult a qualified doctor for medical decisions.