Table of Contents
Overview – Diagnosing Thyroid Dysfunction
Diagnosing thyroid dysfunction involves determining whether the thyroid gland itself or the upstream regulatory centres (hypothalamus and pituitary) are responsible for abnormal hormone levels. A stepwise approach using thyroid function tests, autoantibody assays, dynamic testing, and imaging allows clinicians to differentiate between primary and secondary thyroid disorders such as Hashimoto’s hypothyroidism or Graves’ hyperthyroidism.
Step 1: Localise the Level of Dysfunction
Primary Thyroid Disorders (Thyroid Gland Pathology)
- Primary Hypothyroidism: Thyroid gland fails to produce T3/T4
- Common cause: Hashimoto’s thyroiditis
- Primary Hyperthyroidism: Thyroid gland produces excess T3/T4
- Common cause: Graves’ disease
Secondary Thyroid Disorders (Central Causes)
- Secondary Hypothyroidism: Inadequate TSH stimulation from pituitary or ↓ TRH from hypothalamus
- Secondary Hyperthyroidism: Excess TSH secretion from pituitary (rare)
Step 2: Confirm with Diagnostic Tests
Thyroid Function Tests (TFTs)
- Measure circulating levels of:
- TSH (thyroid-stimulating hormone)
- Free T3 (tri-iodothyronine)
- Free T4 (thyroxine)
Typical Patterns
| Condition | TSH | T3/T4 |
|---|---|---|
| Primary Hypothyroidism | ↑ | ↓ |
| Primary Hyperthyroidism | ↓ | ↑ |
| Secondary Hypothyroidism | ↓ | ↓ |
| Secondary Hyperthyroidism | ↑ | ↑ |
TSH levels are the most sensitive first-line test.
Additional Diagnostic Tools
Thyroid Autoantibody Assays
- Hashimoto’s Thyroiditis:
- Anti-TPO (thyroid peroxidase) antibodies
- Anti-thyroglobulin antibodies
- Graves’ Disease:
- Thyroid-stimulating immunoglobulin (TSI/TsAb)
Note: Thyroid peroxidase is found within follicular cells, while thyroglobulin is stored in the colloid. This explains their staining patterns on histopathology.
TRH Stimulation Test (Dynamic Endocrine Testing)
- Used to assess pituitary TSH response
- Helps diagnose central (secondary) hypothyroidism when TFTs are equivocal
Imaging
- Ultrasound:
- Assesses gland size, vascularity, and nodule presence
- Radioisotope Scan (Scintigraphy):
- Involves uptake of radioactive iodine (I-123 or Tc-99)
- Differentiates:
- “Hot” nodules → increased uptake (hyperfunctioning)
- “Cold” nodules → reduced uptake (risk of malignancy)
Histopathology
- Fine-needle aspiration (FNA) biopsy:
- Used for suspicious nodules
- Confirms malignancy or autoimmune thyroiditis
Summary – Diagnosing Thyroid Dysfunction
Diagnosing thyroid dysfunction involves a structured approach to localise the cause (primary vs secondary) and confirm the underlying pathology through hormone assays, antibodies, imaging, and histopathology. This enables effective identification of conditions like Graves’ disease or Hashimoto’s thyroiditis. For a broader context, see our Endocrine Overview page.