Investigations in thyroid disease

Interpreting thyroid tests and imaging. Rational investigation avoids over-testing minor abnormalities while catching serious disease. TSH is the first-line test.

Classifying thyroid disorders

• Primary — a disorder within the thyroid gland itself (affecting T3/T4 production). The commonest level.
  • Subclinical — T3/T4 within range but TSH abnormal; usually asymptomatic.
  • Overt — T3, T4 and TSH all abnormal; usually symptomatic.
• Secondary — anterior-pituitary disorder affecting TSH secretion.
• Tertiary — hypothalamic disorder affecting TRH, and hence TSH and thyroid hormones.

Antibodies

• Anti-thyroid peroxidase (anti-TPO) and anti-thyroglobulin — anti-TPO is the more commonly detected. Useful in mild/subclinical hypothyroidism: a positive antibody predicts progression to overt disease. No value repeating them (stable over time), and no point in established clinical hypothyroidism (treatment is thyroxine regardless).
• TSH-receptor antibody (TRAb / thyroid-stimulating immunoglobulin) — elevated in Graves’ disease; high titres predict relapse when antithyroid drugs are stopped.

Thyroglobulin

A scaffolding protein storing T3/T4; reflects thyroid tissue mass/inflammation (raised in goitre, hyperthyroidism, thyroiditis). Its main use is as a tumour marker after treatment of papillary/follicular thyroid cancer — it should be undetectable post-thyroidectomy (check 6-monthly initially, then annually). Limited value with an intact gland.

Imaging

• Radionuclide (isotope) scan / scintigraphy — useful in thyrotoxicosis without TRAb (i.e. not Graves’), to find a toxic adenoma or toxic multinodular goitre:
  • Diffuse uptake → Graves’ disease
  • Decreased uptake → thyroiditis
  • Heterogeneous/multifocal → toxic multinodular goitre
  • Focal uptake with suppression elsewhere → hyperfunctioning (toxic) adenoma
• Ultrasound — only useful in patients on amiodarone or with known/palpable nodules. FNA (ultrasound-guided) assesses cystic or solid nodules for carcinoma — see Thyroid cancer.