Thyroid hormone test

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Current biochemistry distinguishes the three common types of hypothyroidism we see in clinical practice namely:

  1. Primary hypothyroidism – where the TSH is near to the limits or above the reference range. The free T4 may be low but often is not.
  2. Secondary hypothyroidism – where the TSH is low. The free T4 may be low, but often is not.
  3. Poor conversion of T4 to T3 – where the most active thyroid hormone, i.e. T3, is low. TSH may be high and the free T4 may be high.

Clinically we often see combinations of the above.

For more detailed information see Thyroid - the correct prescribing of thyroid hormones.