Secondary hypothyroidism

Secondary hypothyroidism is seen when there is interference in

• tyrotropin releasing hormone (TRH) production by the hypothalamus or its delivery via the pituitary stalk
• thyrotropin production in the pituitary

The most common causes are pituitary adenomas and the surgery and/or radiotherapy used to treat them (1).

The disease is diagnosed by a combination of TSH with free T4:

• plasma TSH - can be low, within range or mildly elevated
• free T4 - is low (1)

Patients should be referred to a specialist for further investigations and management. In patients with secondary hypothyroidism, adequacy of thyroxine replacement should be assessed by measuring free T4 levels (1).

Reference:

• 1. Association for Clinical Biochemistry (ACB), British Thyroid Association (BTA), British Thyroid Foundation (BTF) 2006. UK guidelines for the use of thyroid function tests