Decoding Thyroid Function Tests (TSH, Free T4, Free T3)

A practical guide to reading TSH, Free T4 (FT4), and Free T3 (FT3): how they relate, what common result patterns mean, and when to treat, repeat, or look for hidden causes.

Overview

TSH (pituitary signal) inversely tracks circulating thyroid hormone. In primary thyroid disease,TSH moves opposite FT4/FT3; in central (pituitary/hypothalamic) disease, TSH may be inappropriately normal or low with low FT4. Always interpret results with timing, non-thyroidal illness, pregnancy status, and medication effects.

Key Takeaways

Use TSH for screening in stable, non-pregnant adults; reflex to FT4 when TSH is abnormal.
Pattern matters more than any single value; confirm unexpected results by repeating in 6–8 weeks unless urgent.
Drugs, acute illness, and biotin supplements can distort labs; fix confounders before labeling disease.

Physiology in One Minute

The hypothalamus releases TRH → pituitary releases TSH → thyroid makes T4 (prohormone) and T3 (active). Most circulating hormone is protein-bound; we measure free fractions (FT4/FT3) to reflect bioavailable hormone.

Common Interpretation Patterns

High TSH, low FT4: Overt primary hypothyroidism. Treat with levothyroxine; recheck TSH in ~6–8 weeks.
High TSH, normal FT4: Subclinical hypothyroidism. Treat if TSH ≥10 mIU/L, symptomatic, TPO+, pregnant/trying, or cardiovascular risk makes benefit likely.
Low TSH, high FT4/FT3: Overt hyperthyroidism (e.g., Graves, toxic nodules, thyroiditis early phase).
Low TSH, normal FT4/FT3: Subclinical hyperthyroidism; consider age, AF/osteoporosis risk, and cause.
Normal/low-normal TSH, low FT4: Central hypothyroidism—think pituitary/hypothalamus; evaluate adrenal axis before thyroid replacement.
Low TSH, high FT3 with normal FT4: T3-predominant thyrotoxicosis; early Graves or exogenous T3.

When to Order Which Tests

Screening: Start with TSH (non-pregnant, stable). Reflex FT4 if abnormal.
Suspected hyperthyroidism: TSH + FT4; add FT3 if TSH suppressed but FT4 normal.
Confirm autoimmunity: TPO antibodies (Hashimoto). TRAb/TSI if Graves is likely.
Painful thyroid + thyrotoxicosis: Consider ESR/CRP for subacute thyroiditis.
Imaging: Thyroid uptake scan for hyperthyroidism differentiation; ultrasound for nodules/structure—not for routine hypothyroidism.

Special Situations

Pregnancy: Use trimester-specific TSH ranges; treat overt and most subclinical hypothyroidism (TPO+).
Biotin: Stop ≥48 hours before labs—assay interference can falsely lower TSH and raise FT4/FT3.
Illness (“euthyroid sick”): Defer decisions; repeat after recovery unless there’s strong suspicion of true thyroid disease.
Amiodarone, lithium, checkpoint inhibitors: Expect atypical labs; monitor proactively.

Management Snapshots

Levothyroxine: Start ~1.6 mcg/kg/day in healthy adults; lower in elderly/CAD. Take fasting; avoid calcium/iron for 4 hours.
Hyperthyroidism: Beta-block for symptoms. Etiology guides definitive therapy (antithyroid meds, RAI, or surgery).
Follow-up: Recheck TSH (or FT4 in central disease) in 6–8 weeks after any dose change.

Red Flags

Severe thyrotoxicosis with tachyarrhythmia, fever, agitation → consider thyroid storm pathway.
Adrenal insufficiency suspected in central hypothyroidism → evaluate cortisol before thyroid replacement.

Patient FAQs

“How soon will I feel better on levothyroxine?” Energy and cognition usually improve over weeks; full TSH normalization takes 6–8 weeks.

“Can supplements fix thyroid?” Iodine excess can worsen disease; focus on correct diagnosis and evidence-based therapy.

References & Notes

This guide aligns with standard endocrine pathways and major society recommendations. Local lab ranges and protocols vary—use regional guidance and specialist input when needed. Educational only.

Educational only, not personal medical advice.