Thyroid function test: TFT how to interpret TFT and know these dangerous disease at home

Thyroid gland

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Core Tests

• TSH (Thyroid Stimulating Hormone) → most sensitive
• Free T4 (FT4) → active hormone
• Free T3 (FT3) → useful in hyperthyroidism
• ± Antibodies:
  • Anti-TPO
  • TRAb (TSH receptor antibody)

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Step 1: Look at TSH first

          TSH
        /  |   \
     Low Normal High

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IF TSH LOW → Think Hyperthyroid OR Central Hypothyroid

TSH ↓
  |
  ├── FT4 ↑ / FT3 ↑ → PRIMARY HYPERTHYROIDISM
  │        (e.g. Graves', toxic nodules)
  │
  ├── FT3 ↑ only → T3 TOXICOSIS
  │
  ├── FT4 normal, FT3 normal → SUBCLINICAL HYPERTHYROIDISM
  │
  └── FT4 ↓ → CENTRAL (SECONDARY) HYPOTHYROIDISM
           (pituitary/hypothalamic disease)

Diagnoses

• Graves’ disease
• Toxic multinodular goiter
• T3 toxicosis
• Central hypothyroidism

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IF TSH HIGH → Think Hypothyroidism

TSH ↑
  |
  ├── FT4 ↓ → PRIMARY HYPOTHYROIDISM
  │        (thyroid gland failure)
  │
  ├── FT4 normal → SUBCLINICAL HYPOTHYROIDISM
  │
  └── FT4 ↑ → RARE:
           - TSH-secreting adenoma
           - Thyroid hormone resistance

Diagnoses

• Hashimoto thyroiditis
• Iodine deficiency
• Post-thyroidectomy
• TSH-secreting pituitary adenoma

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IF TSH NORMAL → Check FT4/FT3 carefully

TSH normal
  |
  ├── FT4 ↓ → CENTRAL HYPOTHYROIDISM
  │
  ├── FT4 ↑ → TSHOMA / RESISTANCE
  │
  └── FT4 normal → EUTHYROID

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SUMMARY FLOWCHART (HIGH-YIELD)

                TSH
         ┌──────┼──────┐
        ↓       N       ↑
        |       |       |
     Check     Check    Check
     FT4/FT3   FT4      FT4
        |       |        |
   ┌────┼───┐   |    ┌───┼────┐
  ↑     N   ↓   |   ↓    N     ↑
Hyper Subclin Central   Primary Subclin Rare
thyroid hyper  hypo     hypo    hypo   causes

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SPECIAL PATTERNS (EXAM FAVORITES)

1. Euthyroid Sick Syndrome

• TSH: normal/low
• FT3: ↓ (early)
• FT4: ↓ (late)

👉 Seen in severe illness (ICU patients)

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2. Subclinical Disease

Condition	TSH	FT4
Subclinical hyperthyroidism	↓	Normal
Subclinical hypothyroidism	↑	Normal

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3. Pregnancy Changes

• TSH ↓ (1st trimester)
• FT4 slightly ↑ early

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4. Drug Effects

• Amiodarone → hypo OR hyper
• Steroids → ↓ TSH
• Lithium → hypothyroidism

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MEMORY TRICKS

“TSH runs opposite to thyroid hormones”

• ↑ TSH = ↓ thyroid function (usually)
• ↓ TSH = ↑ thyroid function

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“Think CENTRAL if TSH doesn’t match FT4”

• Low TSH + Low FT4 → pituitary problem
• Normal TSH + Low FT4 → pituitary problem

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“Subclinical = TSH abnormal, FT4 normal”

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QUICK CLINICAL PROTOCOL

Suspected Hypothyroidism

1. Check TSH
2. If ↑ → check FT4
3. If FT4 ↓ → treat
4. If FT4 normal → monitor / anti-TPO

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Suspected Hyperthyroidism

1. Check TSH
2. If ↓ → check FT4, FT3
3. If elevated → confirm cause:
   • TRAb → Graves’ disease
   • Scan → nodules

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Suspected Pituitary Disease

• Always check:
  • FT4
  • TSH (may be misleading)

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COMMON PITFALLS

• Relying only on TSH in pituitary disease
• Ignoring FT3 in hyperthyroidism
• Misinterpreting illness-related changes

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ULTRA-SHORT REVISION

• TSH ↑ + FT4 ↓ → Primary hypothyroid
• TSH ↓ + FT4 ↑ → Hyperthyroid
• TSH ↑ + FT4 normal → Subclinical hypo
• TSH ↓ + FT4 normal → Subclinical hyper
• TSH ↓ + FT4 ↓ → Central hypothyroid