Thyroid tests — what TSH, T3, and T4 actually tell you · DiagnoHouse

Hypothyroidism is one of the most common chronic conditions in Indian adults, especially women. Roughly 1 in 10 women over 30 has it — often without knowing. The thyroid profile is the standard screening test, and learning to read it takes about 5 minutes.

What the thyroid does

The thyroid is a butterfly-shaped gland in the front of your neck. It makes hormones that control how fast your body uses energy — metabolism, heart rate, body temperature, even mood. Too little hormone (hypothyroidism) slows everything down. Too much (hyperthyroidism) speeds everything up.

The three numbers on the report

TSH (Thyroid Stimulating Hormone)

This is not a thyroid hormone — it's the brain signal that tells the thyroid to work harder. Normal range: 0.4 to 4.5 mIU/L (the upper limit varies by lab and age).

The clever thing about TSH: when the thyroid is sluggish, the brain pushes harder, so TSH goes up in hypothyroidism. When the thyroid is overactive, the brain stops asking, so TSH goes down in hyperthyroidism.

TSH is the most sensitive single thyroid marker. Even slightly abnormal TSH often points to a real problem before T3 and T4 have visibly shifted.

T4 (Thyroxine)

The main hormone the thyroid makes. Normal range: 4.5 to 12 µg/dL (varies by lab).

The body converts T4 to the more active T3 as needed. T4 is the most stable marker of thyroid output.

T3 (Triiodothyronine)

The more active hormone. Normal range: 0.8 to 2.0 ng/mL.

T3 levels fluctuate more than T4. In early hypothyroidism, T3 may remain normal even when T4 is dropping.

Reading the report — the four common patterns

Pattern 1: TSH high, T3/T4 normal

This is called subclinical hypothyroidism. The thyroid is sluggish but compensating. Whether to treat depends on:

How high TSH is (above 10 is usually treated)
Presence of symptoms (fatigue, weight gain, dry skin)
Whether you're pregnant or planning pregnancy (treated more aggressively)
Anti-thyroid antibody status

Pattern 2: TSH high, T4 low

Overt hypothyroidism. Definitely needs treatment with thyroxine tablets, usually lifelong.

Symptoms: fatigue, weight gain despite normal eating, dry skin, hair fall, cold intolerance, constipation, low mood, slow heart rate, irregular periods.

Pattern 3: TSH low, T3/T4 high

Hyperthyroidism. The thyroid is overproducing.

Symptoms: weight loss despite eating well, palpitations, anxiety, heat intolerance, sweating, tremor, frequent loose stools, irregular periods. Always needs evaluation — common causes include Graves' disease, toxic nodule, or thyroiditis.

Pattern 4: TSH low, T4 normal

Subclinical hyperthyroidism. May be early stage of overactivity, or may follow a temporary thyroiditis. Often retested after 6–8 weeks.

When and how to test

When to test

Annual screening for women aged 30+ (especially with family history)
Unexplained fatigue, weight changes, hair fall, low mood
Irregular periods or fertility problems
During pregnancy (first trimester and as advised)
6–8 weeks after starting or changing thyroxine dose

How to test

No fasting needed — the result is the same whether you've eaten or not.
Morning slot preferred. TSH naturally rises in the early morning and drops through the day. Consistent morning sampling makes serial tests comparable.
If you're on thyroxine, take it AFTER the blood draw, not before. Taking it before falsely raises T4 readings.
Pause biotin supplements for 48 hours. Biotin interferes with the assay.

Living with hypothyroidism

For most people, hypothyroidism is straightforward to manage:

Daily tablet of levothyroxine, taken first thing in the morning on an empty stomach, 30–60 minutes before food.
TSH check every 6–8 weeks initially while finding the right dose. Once stable, every 6–12 months.
Most patients feel back to normal within 2–3 months of the right dose.
Calcium supplements, iron supplements, antacids, and soy can interfere with absorption — take them 4 hours apart from your thyroxine.

Why is hypothyroidism so common in Indians?

A combination of factors:

Iodine sufficiency has improved with iodised salt, but pockets of deficiency still exist.
Autoimmune thyroiditis (Hashimoto's) is the most common cause and has a genetic component — runs in families.
Vitamin D and B12 deficiency are widespread and often coexist with thyroid problems.
Pregnancy and post-partum can trigger hypothyroidism in susceptible women.

Annual screening is cheap and the consequences of missed hypothyroidism (heart disease, depression, fertility problems) are real. Worth the ₹550 once a year.

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Thyroid Profile (T3, T4, TSH)