Lab Interpretation Guide
Thyroid Function Tests — Complete Guide
TSH · Free T4 · Free T3 · Anti-TPO · TRAb · Hypothyroidism & Hyperthyroidism
Quick Answer
TSH is the most sensitive first-line test for thyroid dysfunction. Elevated TSH indicates hypothyroidism; suppressed TSH indicates hyperthyroidism. Free T4 and Free T3 confirm the diagnosis and assess severity. Anti-TPO identifies autoimmune thyroid disease.
Reference Ranges
Marker
Normal range
Clinical note
TSH
0.4–4.0 mIU/L
Pregnancy: 0.1–2.5 mIU/L (1st trimester). Most sensitive thyroid test.
Free T4
12–22 pmol/L
Active thyroid hormone. Low in hypothyroidism, high in hyperthyroidism.
Free T3
3.1–6.8 pmol/L
Most biologically active form. Rises early in hyperthyroidism.
Anti-TPO
< 34 IU/mL
Marker of autoimmune thyroid disease (Hashimoto's, Graves).
Anti-TG
< 115 IU/mL
Less specific than Anti-TPO. Elevated in Hashimoto's and thyroid cancer monitoring.
TRAb
< 1.75 IU/L
Graves disease marker — TSH receptor antibody. Stimulates thyroid hormone production.
TSH0.4–4.0 mIU/L
Pregnancy: 0.1–2.5 mIU/L (1st trimester). Most sensitive thyroid test.
Free T412–22 pmol/L
Active thyroid hormone. Low in hypothyroidism, high in hyperthyroidism.
Free T33.1–6.8 pmol/L
Most biologically active form. Rises early in hyperthyroidism.
Anti-TPO< 34 IU/mL
Marker of autoimmune thyroid disease (Hashimoto's, Graves).
Anti-TG< 115 IU/mL
Less specific than Anti-TPO. Elevated in Hashimoto's and thyroid cancer monitoring.
TRAb< 1.75 IU/L
Graves disease marker — TSH receptor antibody. Stimulates thyroid hormone production.
Interpretation Patterns
TSH ↑↑+FT4 ↓→Overt Hypothyroidism
Primary thyroid failure. Fatigue, weight gain, cold intolerance, bradycardia, dry skin. Levothyroxine indicated.
TSH ↑+FT4 normal→Subclinical Hypothyroidism
Treat if TSH > 10, symptomatic, Anti-TPO positive, or pregnant. Monitor annually if TSH 4–10.
TSH ↓↓+FT4 ↑→Overt Hyperthyroidism
Weight loss, tachycardia, tremor, heat intolerance, anxiety, AF. Graves disease or toxic nodule. Antithyroid therapy required.
TSH ↓+FT4 normal→Subclinical Hyperthyroidism
Monitor for atrial fibrillation and bone density loss. Treat if TSH < 0.1 persistently or age > 65.
TSH ↑+FT4 ↑→Discordant — Endocrinology
Rare pattern. Consider TSH-secreting pituitary adenoma, thyroid hormone resistance, or assay interference (biotin excess).
TSH ↓+FT4 ↓→Central Hypothyroidism
Pituitary or hypothalamic failure — TSH is low despite low thyroid hormones. Rare. Pituitary MRI indicated.
Autoimmune Thyroid Disease
Hashimoto's Thyroiditis
Anti-TPO ↑↑ + Anti-TG ↑
Most common cause of hypothyroidism. TSH rises progressively. Annual monitoring. Levothyroxine when TSH > 10 or symptomatic.
Graves' Disease
TRAb ↑ + Anti-TPO ↑
Most common cause of hyperthyroidism. Diffuse goiter, exophthalmos. Methimazole or radioiodine treatment.
Subclinical Autoimmunity
Anti-TPO ↑ + TSH normal
4× increased lifetime risk of hypothyroidism. Annual TSH monitoring recommended. No treatment unless symptomatic.
Key Clinical Points
- •TSH has a logarithmic relationship with thyroid hormones — a small TSH change reflects a large hormone shift
- •In pregnancy, TSH target is 0.1–2.5 mIU/L in first trimester — critical for fetal neurodevelopment
- •Anti-TPO positive + normal TSH = 4× increased risk of developing hypothyroidism — monitor annually
- •Sick euthyroid syndrome: TSH and T4 both low in acute illness without true thyroid disease — do not treat
- •Biotin supplements > 5 mg/day cause falsely abnormal thyroid results — stop 2 days before testing
- •Amiodarone causes both hypothyroidism and hyperthyroidism — thyroid monitoring every 6 months required
- •Free T3 elevation with normal FT4 and suppressed TSH = T3 toxicosis — often toxic multinodular goiter
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