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TSH (Thyroid Stimulating Hormone)vsFT4 (Free Thyroxine)

TSH vs Free T4

Understanding the relationship between TSH and Free T4 in thyroid function testing, and why both are needed for accurate assessment.

Aspect
TSH (Thyroid Stimulating Hormone)
FT4 (Free Thyroxine)
What it measures
Thyroid Stimulating Hormone - a pituitary hormone that signals the thyroid to produce hormones
Free Thyroxine - the unbound, active form of the main thyroid hormone T4
Source
Produced by the pituitary gland in the brain
Produced by the thyroid gland in the neck
Interpretation
High TSH = thyroid underproducing (hypothyroid); Low TSH = thyroid overproducing (hyperthyroid)
Low Free T4 = hypothyroid; High Free T4 = hyperthyroid
Sensitivity
Most sensitive marker for thyroid dysfunction - changes before T4 in most cases
Confirms thyroid hormone production level - may lag behind TSH changes
When used alone
Good for screening but can miss pituitary problems and some thyroid conditions
Insufficient alone - needs TSH context to interpret correctly

Key Differences

  • •TSH is a "signal" from the brain; Free T4 is the actual thyroid hormone
  • •TSH rises when thyroid output is low (inverse relationship)
  • •TSH changes first in most thyroid disorders, making it the most sensitive screening test
  • •Free T4 tells you what the thyroid is actually producing
  • •Both are needed to distinguish between different types of thyroid disorders

Why Both Matter

TSH and Free T4 work together in a feedback loop that maintains thyroid balance. The pituitary constantly monitors thyroid hormone levels; when Free T4 drops, TSH rises to stimulate more production, and vice versa.

TSH is the most sensitive screening test because it typically changes before Free T4 becomes abnormal. In early thyroid failure, TSH may be elevated while Free T4 remains in the normal range (subclinical hypothyroidism). This is why TSH is the standard first-line test.

However, TSH alone can miss important conditions: - Central hypothyroidism (pituitary problems) where both TSH and Free T4 are low - Patients on thyroid medication where Free T4 helps optimize dosing - Conditions where TSH-Free T4 relationship is altered (non-thyroidal illness, certain medications)

For comprehensive thyroid assessment, especially in patients with symptoms or on treatment, measuring both TSH and Free T4 (and often Free T3) provides the complete picture needed for accurate diagnosis and optimal treatment.

Learn more about

TSH (Thyroid Stimulating Hormone)

A pituitary hormone that regulates thyroid function.

Learn more about

FT4 (Free Thyroxine)

The unbound, biologically active form of T4, the main thyroid hormone.

Frequently Asked Questions

Why is TSH high when my thyroid is underactive?
TSH works like a thermostat. When the thyroid underproduces hormones, the pituitary releases more TSH to try to stimulate production. High TSH is the body's attempt to fix low thyroid output.
Can I have thyroid problems with normal TSH?
Yes. Central hypothyroidism (pituitary problems) causes low TSH and low Free T4. Some people also have symptoms with TSH in the "normal" range, especially 2.5-4.5. Full panel testing can reveal these cases.
Which test is more important for monitoring thyroid medication?
Both are important. TSH shows whether your dose achieves the right "set point." Free T4 shows actual hormone levels. Together they help optimize your dose and timing.
How quickly do TSH and Free T4 respond to treatment changes?
Free T4 responds within days to dose changes. However, TSH takes 6-8 weeks to fully stabilize, which is why testing is typically done 6-8 weeks after dose adjustments.

Track Both Markers Over Time

Upload your blood test results to BloodTrack and monitor both TSH (Thyroid Stimulating Hormone) and FT4 (Free Thyroxine). See how they change together over time.

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