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Thyroid Function Tests (TFT) - Complete Explanation

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1. The Hypothalamic-Pituitary-Thyroid (HPT) Axis - The Foundation

Hypothalamus  →  TRH (Thyrotropin-Releasing Hormone)
      ↓
Anterior Pituitary  →  TSH (Thyroid-Stimulating Hormone)
      ↓
Thyroid Gland  →  T4 (Thyroxine) + T3 (Triiodothyronine)
      ↓
Negative Feedback back to Hypothalamus & Pituitary

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2. TSH (Thyroid-Stimulating Hormone)

What it is

• A glycoprotein secreted by the anterior pituitary
• Has alpha and beta subunits - the beta subunit carries specificity for the TSH receptor
• Normal range: 0.35 - 5.0 mIU/L (most labs); some use 0.5-5 μU/mL

Why it is the first-line test

• A small change in free T4 causes a large, amplified change in TSH
• This makes TSH the most sensitive and specific single test for thyroid dysfunction
• A normal TSH virtually excludes primary thyroid disease in a stable, otherwise healthy patient

Assay evolution (generations)

• Frank Graves hyperthyroidism: TSH <0.01 mIU/L
• Subclinical hyperthyroidism: TSH 0.01-0.1 mIU/L

Key limitations of TSH alone

• Secondary (pituitary) hypothyroidism: TSH is low/normal despite low T4 - TSH is inappropriate here
• Tertiary (hypothalamic) hypothyroidism: same problem
• Non-thyroidal illness (NTI/sick euthyroid): TSH suppressed during acute illness, rebounds above normal in recovery
• Medications: glucocorticoids, dopamine, opioids all suppress TSH
• Biotin interference: supplements >1000 mcg cause falsely LOW TSH (stop biotin 2 days before testing)
• Recent thyroid therapy: TSH remains suppressed for months after treating hyperthyroidism

Source: Harrison's Principles of Internal Medicine 22E; Henry's Clinical Diagnosis and Management by Laboratory Methods

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3. Understanding Bound vs Free Hormones - The Critical Concept

Thyroid Hormone Binding Proteins

• Enters target cells
• Binds nuclear thyroid hormone receptors
• Exerts metabolic effects
• Provides negative feedback to the pituitary

"Only a small fraction of the hormones are free and unbound. Laboratory measurement of total T3 and total T4 measures mainly protein-bound hormone concentrations." - Tintinalli's Emergency Medicine

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4. Total T4 (Serum Thyroxine)

What it measures

• Both free + protein-bound T4 combined (predominantly bound)
• Measured by radioimmunoassay (RIA)
• Normal range: 55-150 nmol/L (or ~4.5-12 μg/dL)

What it reflects

• Primarily reflects thyroid gland output (T4 secretion)
• Since T4 is ~99.97% protein-bound, total T4 fluctuates with changes in binding protein levels, not just thyroid function

Factors that raise Total T4 (without true hyperthyroidism)

• Pregnancy (elevated estrogen raises TBG)
• Oral contraceptives / estrogen therapy
• Tamoxifen, SERMs
• Inflammatory liver disease
• Hereditary TBG excess
• Hyperemesis gravidarum (hCG cross-stimulates thyroid)

Factors that lower Total T4 (without true hypothyroidism)

• Anabolic steroids / androgens (reduce TBG)
• Nephrotic syndrome (protein loss)
• Malnutrition
• Drugs: phenytoin, carbamazepine, salicylates, NSAIDs (displace T4 from protein binding)

Source: Schwartz's Principles of Surgery 11E; Harrison's Principles of Internal Medicine 22E

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5. Total T3 (Serum Triiodothyronine)

What it measures

• Free + bound T3 combined
• Normal range: 1.5-3.5 nmol/L (or ~80-220 ng/dL)

Why it differs from Total T4

• T3 is the biologically active form (3-4x more potent than T4)
• 80% of circulating T3 comes from peripheral deiodination of T4, not direct thyroid secretion
• Therefore T3 levels reflect peripheral thyroid hormone metabolism more than gland output
• Total T3 is not suitable as a general screening test for this reason

When Total T3 is specifically useful

1. T3 thyrotoxicosis: ~2-5% of hyperthyroid patients have isolated elevated T3 with normal T4 - classic in early Graves' disease or toxic nodule
2. Early hyperthyroidism: T3 rises before T4
3. TSH-secreting tumors: both T3 and T4 elevated with non-suppressed TSH

An important counterintuitive fact

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6. Free T4 (FT4)

What it measures

• Only the unbound, biologically active T4 fraction
• Normal range: 12-28 pmol/L (or ~0.7-1.9 ng/dL)
• Measured by: direct immunoassay (equilibrium dialysis is gold standard), or calculated via Free T4 Index

Why it is better than Total T4

• NOT affected by changes in binding protein levels
• Directly reflects the hormone available to tissues and the pituitary
• Correlates directly with the pituitary's "perception" of thyroid status

Clinical use of Free T4

• Confirms hyperthyroidism when TSH is suppressed
• Confirms hypothyroidism when TSH is elevated
• Essential in pituitary/hypothalamic disease (where TSH is unreliable)
• Detects early hyperthyroidism: FT4 rises before Total T4
• End-organ resistance (Refetoff's syndrome): T4 elevated but TSH normal - FT4 helps clarify

Free T4 Index (FT4I) - indirect method

"An indirect method that is now less commonly used to estimate unbound thyroid hormone levels is to calculate the free T3 or free T4 index from the total T4 or T3 concentration and the thyroid hormone binding ratio (THBR)." - Harrison's 22E

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7. Free T3 (FT3)

What it measures

• Unbound, active T3 fraction
• Normal range: 3-9 pmol/L (or ~2.3-4.2 pg/mL)

When it is specifically used

• Most useful in confirming early hyperthyroidism: FT3 and FT4 rise before total T4 and total T3
• T3 toxicosis: suppressed TSH + normal FT4 → check FT3 (will be elevated)
• Monitoring Graves' disease treatment
• Not useful in hypothyroidism (stays normal until late)

"For most purposes, the unbound T4 level is sufficient to confirm thyrotoxicosis, but 2-5% of patients have only an elevated T3 level (T3 toxicosis). Thus, unbound T3 levels should be measured in patients with a suppressed TSH but normal unbound T4 levels." - Harrison's Principles of Internal Medicine 22E

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8. The Relationship Between Total and Free Hormones - Summary

Total T4/T3  =  Bound (inactive) fraction  +  Free (active) fraction
                     (~99.97% / ~99.7%)           (~0.03% / ~0.3%)

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9. TFT Pattern Interpretation - Clinical Summary

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10. Practical Order of Testing

1. Step 1 - Always start with TSH: Most sensitive single screening test
2. Step 2 - If TSH is abnormal: Measure FT4 (and FT3 if TSH suppressed but FT4 normal)
3. Step 3 - Total T4/T3: Used when binding protein abnormalities are suspected, or in specific scenarios like T3 toxicosis screening
4. Free T4 Index: Used when direct FT4 immunoassay may be unreliable (e.g., pregnancy, critical illness)
5. TRH stimulation test: Now largely obsolete due to highly sensitive TSH assays

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Key Teaching Points

• TSH is the master test - amplified, logarithmic response makes it the most sensitive indicator
• Free hormones = biologically active - they reflect actual tissue hormone exposure
• Total hormones = mostly bound - they change with protein levels, not just thyroid function
• T4 is the thyroid's main product; T3 is the main active hormone produced peripherally
• Never rely on Total T4/T3 alone when binding protein levels may be abnormal
• FT3 is the most specific test for T3 thyrotoxicosis and early hyperthyroidism
• In pituitary/hypothalamic disease, do not use TSH alone - always add FT4

Sources: Harrison's Principles of Internal Medicine 22E, p.3069; Schwartz's Principles of Surgery 11E; Henry's Clinical Diagnosis and Management by Laboratory Methods; Tintinalli's Emergency Medicine