SN7
SHORT NOTE
5M
Enumerate the thyroid function tests and describe their interpretation.
Purpose
Thyroid function tests assess the functional state of the hypothalamo–pituitary–thyroid axis and help distinguish primary from secondary disease.
Hormone Assays
| Test | Normal value | Comment |
|---|---|---|
| TSH | 0.5–5 mIU/L | The single most sensitive test for primary thyroid disease; the first to change |
| Free T4 | 0.8–2.0 ng/dL | Preferred over total T4 — unaffected by binding proteins |
| Free T3 | 2.3–4.2 pg/mL | Useful in T3 toxicosis |
| Total T4 | 5–12 µg/dL | Altered by pregnancy and oral contraceptives (↑ TBG) |
| Total T3 | 80–200 ng/dL | Same limitation |
Interpretation Patterns
| TSH | Free T4 | Diagnosis |
|---|---|---|
| ↑ | ↓ | Primary hypothyroidism |
| ↑ | Normal | Subclinical hypothyroidism |
| ↓ | ↑ | Primary hyperthyroidism |
| ↓ | Normal | Subclinical hyperthyroidism |
| ↓ or normal | ↓ | Secondary (pituitary) hypothyroidism |
| ↑ | ↑ | TSH-secreting adenoma or thyroid hormone resistance |
Antibody Tests
- Anti-TPO and anti-thyroglobulin → Hashimoto thyroiditis
- TSH receptor antibody (TRAb) → Graves disease
Other Investigations
| Test | Use |
|---|---|
| Radioactive iodine uptake | High in Graves disease; low in thyroiditis and factitious thyrotoxicosis |
| Radioisotope scan | Hot nodule — usually benign; cold nodule — 15–20% malignant |
| FNAC | The investigation of choice for a thyroid nodule |
| Ultrasound | Size, cystic vs solid, lymph nodes |
| Serum thyroglobulin | Follow-up marker after thyroid cancer surgery |
| Serum calcitonin | Marker of medullary carcinoma |
Applied Aspects
- Sick euthyroid syndrome — in severe non-thyroidal illness, T3 falls with normal TSH. Do not test thyroid function in acutely ill patients unless thyroid disease is genuinely suspected
- Pregnancy — oestrogen raises TBG, so total T4 rises while free T4 stays normal; use trimester-specific TSH ranges
- Amiodarone can cause either hypo- or hyperthyroidism because of its high iodine content
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- SN7Enumerate the thyroid function tests and describe their interpretation.