Central Hypothyroidism

Central Hypothyroidism: Causes, Diagnosis, and Why TSH Alone Can Miss It

Central hypothyroidism is an underactive thyroid caused by a problem in the pituitary gland or hypothalamus rather than in the thyroid itself. The thyroid is capable of working normally — it is simply not receiving the signal to do so. That single difference changes how the condition is found, how it is confirmed, and which blood test is used to guide treatment.

What is central hypothyroidism?

In ordinary (primary) hypothyroidism, the thyroid gland underperforms and the pituitary responds by raising TSH. In central hypothyroidism the pituitary or hypothalamus is the source of the problem, so that TSH rise does not happen reliably. The pattern is:

Low free T4
TSH that is low, normal, or only mildly raised
Usually in the context of known or suspected pituitary or hypothalamic disease

The Endocrine Society states that a free T4 below the laboratory reference range, together with a low, normal, or mildly elevated TSH in the setting of pituitary disease, usually confirms the diagnosis. UK thyroid guidance puts it the same way: TSH in these patients "can be low, within or mildly above the reference range."

Why TSH-first screening can miss it

Most thyroid screening starts with TSH and only measures free T4 if TSH is abnormal. That strategy is built for primary hypothyroidism and can miss central hypothyroidism, because the TSH may come back inside the reference range — or even slightly above it, which points the interpretation in exactly the wrong direction.

This is the main reason free T4 is measured alongside TSH, rather than only after it, when pituitary disease is suspected.

What causes it?

1. Pituitary tumors. A pituitary adenoma, or pressure from a nearby mass, can reduce TSH production.

2. Pituitary surgery or radiotherapy. Treatment of a pituitary or nearby brain tumor can leave lasting deficiency.

3. Head trauma or subarachnoid hemorrhage. Injury to the pituitary or its blood supply.

4. Infiltrative and inflammatory disease. Conditions such as hypophysitis, sarcoidosis, hemochromatosis or histiocytosis affecting the pituitary or hypothalamus.

5. Pregnancy-related pituitary injury. Including Sheehan's syndrome after severe postpartum hemorrhage.

6. Congenital and genetic causes. Isolated TSH deficiency or combined pituitary hormone deficiency.

Because these causes often affect more than one pituitary hormone, central hypothyroidism is frequently found alongside other deficiencies — of cortisol, growth hormone or the sex hormones — and cortisol status in particular is assessed before thyroid hormone replacement is started.

How it is treated and monitored

Treatment is levothyroxine, as in primary hypothyroidism. The monitoring is different, and this is the part most often got wrong. The Endocrine Society recommends dosing to a free T4 in the mid to upper half of the reference range, and recommends against using TSH to adjust replacement in central hypothyroidism — because the TSH that would normally report under-replacement is the very thing that is not working.

So in central hypothyroidism, free T4 is the dose marker and TSH is not.

Central Hypothyroidism