Unit Converter
TSH – Thyroid‑stimulating hormone
(Primary Screening Test for Thyroid Dysfunction - Pituitary Glycoprotein Hormone Regulating T4 & T3)
Synonyms
- Thyrotropin
- TSH
- Thyroid-stimulating hormone
- Pituitary thyrotropin
- Thyrotropic hormone
Units of Measurement
- µIU/mL
- mIU/L
(Both units are numerically equivalent - see below.)
Unit Conversions
µIU/mL ↔ mIU/L
1 µIU/mL=1 mIU/L1\ \text{µIU/mL} = 1\ \text{mIU/L}1 µIU/mL=1 mIU/L
TSH is universally reported in mIU/L or µIU/mL - no mass units.
Description
TSH is a pituitary glycoprotein hormone composed of:
- α-subunit (common with LH, FSH, hCG)
- β-subunit (TSH-specific)
TSH regulates:
- Thyroid gland growth
- Thyroid hormone synthesis
- Secretion of T4 and T3
It is the most sensitive marker of thyroid gland function because of its log–linear inverse relationship with circulating free T4.
Even tiny reductions in FT4 cause large increases in TSH.
Physiological Role
TSH binds to TSH receptors on thyroid follicular cells and stimulates:
- Iodide uptake
- Thyroglobulin iodination
- T4/T3 synthesis
- Thyroid growth (trophic effect)
Regulated by:
- Negative feedback from FT4 and FT3
- Hypothalamic TRH
- Circadian rhythm (peak 2–4 AM)
Clinical Significance
ELEVATED TSH
Indicates low thyroid hormone activity.
Primary Hypothyroidism
- Hashimoto thyroiditis (most common)
- Iodine deficiency
- Post-surgery or radioiodine
- Congenital hypothyroidism
- Severe thyroiditis (late phase)
Lab pattern:
- TSH ↑
- FT4 ↓
Subclinical Hypothyroidism
- TSH ↑
- FT4 normal
Seen in early autoimmune disease.
Medications Elevating TSH
- Lithium
- Amiodarone
- Interferon-α
- Anti-thyroid drugs
- TKIs (tyrosine kinase inhibitors)
TSH Surges
- Pregnancy (rare)
- Recovery from non-thyroidal illness
LOW TSH
Indicates excess thyroid hormone activity or pituitary suppression.
Primary Hyperthyroidism
- Graves’ disease
- Toxic MNG
- Toxic adenoma
Lab pattern:
- TSH ↓
- FT4 ↑ and/or FT3 ↑
Subclinical Hyperthyroidism
- TSH ↓
- FT4/FT3 normal
Central Hypothyroidism
Pituitary or hypothalamic disease:
- TSH low or inappropriately normal
- FT4 ↓
Non-thyroidal Illness Syndrome
TSH may be:
- mildly reduced
- normal
- transiently high in recovery phase
Medications Lowering TSH
- Glucocorticoids
- Dopamine
- Octreotide
- Carbamazepine
- High-dose biotin (false changes in immunoassays)
Reference Intervals
(Tietz 8E + ATA + Mayo + ARUP)
Standard TSH Reference Range
- 0.4 – 4.0 mIU/L (most common)
Some labs use: - 0.5 – 4.5 mIU/L
Pregnancy-Specific Ranges
- 1st trimester: 0.1 – 2.5 mIU/L
- 2nd trimester: 0.2 – 3.0 mIU/L
- 3rd trimester: 0.3 – 3.0 mIU/L
Neonates
- At birth: up to 60–80 mIU/L (physiological surge)
- By 1 week: falls to <10 mIU/L
Diagnostic Uses
1. Primary Test for Thyroid Screening
- First-line test in adults
- Detects subtle thyroid dysfunction
2. Monitoring Hypothyroidism Treatment
Levothyroxine dose adjustments guided by TSH.
3. Evaluating Hyperthyroidism
Low TSH prompts measurement of FT4, FT3.
4. Newborn Screening
Detects congenital hypothyroidism.
5. Assessing Pituitary Function
Low/inappropriately normal TSH with low FT4 → central hypothyroidism.
6. Medication Monitoring
Drugs affecting TSH axis.
Analytical Notes
- TSH measured by third-generation chemiluminescent immunoassays (functional sensitivity ~0.01 mIU/L).
- Biotin interference can alter TSH levels - high-dose biotin may falsely lower TSH.
- Severe illness alters thyroid hormone binding → interpret cautiously.
Clinical Pearls
- A normal TSH almost always rules out primary thyroid disease.
- TSH may remain suppressed for weeks after hyperthyroidism treatment despite normalized FT4/FT3.
- Elderly may target TSH 4–6 mIU/L for LT4 therapy to avoid overtreatment.
- In pregnancy, always interpret TSH with trimester-specific cutoffs.
- Central hypothyroidism → TSH useless; rely on FT4.
Interesting Fact
TSH operates on a log–linear feedback curve:
A tiny change in FT4 causes a large change in TSH, making it the most sensitive hormone test in endocrinology.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Thyroid Function Testing
- ATA/AACE Clinical Practice Guidelines for Hypothyroidism & Hyperthyroidism
- European Thyroid Association (ETA)
- Mayo Clinic Laboratories - TSH
- ARUP Consult - Thyroid Function Tests
- NIH / MedlinePlus - TSH Test