Table of Contents
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)
Step 1: Look at TSH first
TSH
/ | \
Low Normal High
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
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
IF TSH NORMAL → Check FT4/FT3 carefully
TSH normal
|
├── FT4 ↓ → CENTRAL HYPOTHYROIDISM
│
├── FT4 ↑ → TSHOMA / RESISTANCE
│
└── FT4 normal → EUTHYROID
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
SPECIAL PATTERNS (EXAM FAVORITES)
1. Euthyroid Sick Syndrome
- TSH: normal/low
- FT3: ↓ (early)
- FT4: ↓ (late)
👉 Seen in severe illness (ICU patients)
2. Subclinical Disease
| Condition | TSH | FT4 |
|---|---|---|
| Subclinical hyperthyroidism | ↓ | Normal |
| Subclinical hypothyroidism | ↑ | Normal |
3. Pregnancy Changes
- TSH ↓ (1st trimester)
- FT4 slightly ↑ early
4. Drug Effects
- Amiodarone → hypo OR hyper
- Steroids → ↓ TSH
- Lithium → hypothyroidism
MEMORY TRICKS
“TSH runs opposite to thyroid hormones”
- ↑ TSH = ↓ thyroid function (usually)
- ↓ TSH = ↑ thyroid function
“Think CENTRAL if TSH doesn’t match FT4”
- Low TSH + Low FT4 → pituitary problem
- Normal TSH + Low FT4 → pituitary problem
“Subclinical = TSH abnormal, FT4 normal”
QUICK CLINICAL PROTOCOL
Suspected Hypothyroidism
- Check TSH
- If ↑ → check FT4
- If FT4 ↓ → treat
- If FT4 normal → monitor / anti-TPO
Suspected Hyperthyroidism
- Check TSH
- If ↓ → check FT4, FT3
- If elevated → confirm cause:
- TRAb → Graves’ disease
- Scan → nodules
Suspected Pituitary Disease
- Always check:
- FT4
- TSH (may be misleading)
COMMON PITFALLS
- Relying only on TSH in pituitary disease
- Ignoring FT3 in hyperthyroidism
- Misinterpreting illness-related changes
ULTRA-SHORT REVISION
- TSH ↑ + FT4 ↓ → Primary hypothyroid
- TSH ↓ + FT4 ↑ → Hyperthyroid
- TSH ↑ + FT4 normal → Subclinical hypo
- TSH ↓ + FT4 normal → Subclinical hyper
- TSH ↓ + FT4 ↓ → Central hypothyroid
