Key Laboratory Ratios and Reference Values for Thyroid and Liver Assessment
For thyroid function, TSH (0.66-5.84 mIU/L) and free T4 (8.62-14.71 pmol/L) are the essential tests, while for liver assessment, ALT, ALP, GGT, bilirubin, and albumin form the core panel, with no single "ratio" being standard but rather patterns of elevation guiding diagnosis. 1, 2
Thyroid Function Testing
Primary Tests and Reference Values
TSH is the single most important screening test for thyroid dysfunction 3, 4:
- Male reference range: 0.66-4.95 mIU/L
- Female reference range: 0.72-5.84 mIU/L 2
Free T4 (FT4) reference values 2:
- Male: 8.97-14.71 pmol/L
- Female: 8.62-14.35 pmol/L
Free T3 (FT3) reference values 2:
- Male: 3.75-5.81 pmol/L
- Female: 3.59-5.56 pmol/L
Diagnostic Thresholds for Thyroid Disease
Subclinical hypothyroidism: TSH 4.5-10 mIU/L with normal FT4 3
Overt hypothyroidism: TSH >10 mIU/L with low FT4, or high TSH with low-normal FT4 plus symptoms 3
Subclinical hyperthyroidism: TSH <0.4 mIU/L (or more specifically <0.1 mIU/L) with normal FT4/T3 3
Overt hyperthyroidism: TSH <0.1 mIU/L with elevated FT4, T3, or FT3 3
Important Thyroid Ratios (Emerging Clinical Utility)
The T3/T4 ratio is gaining recognition as a marker of thyroid hormone conversion and tissue-level thyroid status, though not yet standard practice. This ratio may provide additional information about metabolic consequences even in biochemically euthyroid patients 5.
Critical Pitfall: Chronic Liver Disease
In patients with chronic liver disease, TSH alone is the most reliable test 4. Total T4, T3, and binding protein measurements are frequently abnormal despite clinical euthyroidism due to:
- Altered thyroxine binding globulin levels
- Reduced T3 levels correlating with disease severity
- Elevated FT4 with low-normal FT3 maintaining euthyroid state 6
The simplified approach: Use TSH immunoradiometric assay (IRMA) as a single test in chronic liver disease patients—it removes uncertainties from binding protein abnormalities 4.
Liver Function Testing
Standard Liver Blood Test Panel
The essential initial panel includes 1:
- Bilirubin
- Albumin
- ALT (alanine aminotransferase)
- ALP (alkaline phosphatase)
- GGT (gamma-glutamyl transferase)
- Full blood count (if not done within 12 months)
Key Patterns Rather Than Ratios
Unlike thyroid testing, liver assessment relies on patterns of enzyme elevation rather than specific ratios:
Hepatocellular pattern: Predominantly elevated ALT/AST (>5x upper limit suggests acute hepatocellular injury)
Cholestatic pattern: Predominantly elevated ALP/GGT
Mixed pattern: Both hepatocellular and cholestatic markers elevated
Important Liver-Related Indices
AST/ALT ratio: Though not mentioned in the guideline, this is clinically useful:
- Ratio >2 suggests alcoholic liver disease
- Ratio <1 typical in viral hepatitis or NAFLD
Platelet count: Low platelets indicate advanced fibrosis/cirrhosis due to splenic sequestration from portal hypertension 1
Non-Invasive Fibrosis Markers
FIB-4 Index (Fibrosis-4): Uses age, AST, ALT, and platelet count to assess liver fibrosis 5
FLI (Fatty Liver Index): Assesses hepatic steatosis 5
Extended Liver Aetiology Panel
When initial tests are abnormal, proceed with 1:
- Viral hepatitis: HBsAg, HCV antibody (with PCR if positive)
- Iron overload: Ferritin AND transferrin saturation
- Autoimmune: AMA, ASMA, ANA, immunoglobulins
- Metabolic: Alpha-1-antitrypsin, thyroid function tests, caeruloplasmin (age 3-40 years)
Thyroid-Liver Interaction
In NAFLD patients, elevated TSH (even within normal range) and elevated neutrophil-to-lymphocyte ratio (NLR) correlate with disease severity 7. High-normal FT3 and low-normal TSH predict incident NAFLD in euthyroid subjects 8.
Clinical Algorithm
- Screen with TSH for thyroid dysfunction (add FT4 if TSH abnormal)
- Screen with ALT, ALP, GGT, bilirubin, albumin for liver disease
- If chronic liver disease present: Use TSH-IRMA alone for thyroid assessment
- If both abnormal: Consider thyroid-liver axis interaction—thyroid dysfunction can worsen liver disease and vice versa
Age considerations: FT4, FT3, and total T3 decline with age; adjust interpretation accordingly 2.