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Thyroid Hormone Levels

Measuring the thyroid stimulating hormone (TSH) levels is used to test thyroid function.#

  • A high TSH level suggests that the thyroid gland is not producing enough thyroid hormone (primary hypothyroidism).
  • A low TSH level usually suggest that the thyroid is producing too much thyroid hormone (hyperthyroidism).
  • The total thyroxine (T4) hormone measures both bound and free thyroxine levels in the blood.
  • Free T4 (FT4) measures the free thyroxine hormone, which is not bound and able to freely enter and affect body tissues.
  • Triiodothyronine (T3) levels measures the bound and free fractions of triiodothyronine.

Adult range of thyroid hormone levles#

Type of hormoneNormal levels
TSH0.5 - 5.0 mIU/L
T480 - 220 ng/dL
FT40.7 - 1.9 ng/dL
T35.0 - 12.0 μg/dL
### Differential diagnosis of a thyroid stimulating hormone (TSH) test
Laboratory resultsDifferential Diagnosis
High TSH, low thyroid hormone levelPrimary hypothyroidism
High TSH, normal thyroid hormone levelSubclinical hypothyroidism
Low TSH, high thyroid hormone levelPrimary hyperthyroidism
Low TSH, normal thyroid hormone levelEarly or mild hyperthyroidism
Low TSH, high thyroid hormone level
and subsequent…
High TSH, low thyroid hormone level
Thyroiditis (Thyroid Inflammation)
Low TSH, low thyroid hormone levelPituitary disease

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