What does a low free T4 with a normal TSH mean?

In the usual underactive thyroid, the thyroid fails and the pituitary pushes back with more TSH. The 2012 AACE and ATA hypothyroidism guideline describes TSH secretion as exquisitely sensitive to small changes in free T4, which is why TSH is the first-line test. It then draws the distinction this result turns on: a subnormal free T4 establishes hypothyroidism "whether primary, in which serum TSH is elevated, or central, in which serum TSH is normal or low."

So a low free T4 next to a normal TSH means one of two things in these guidelines. Either the free T4 number is low for a reason other than thyroid failure, or the signal from the pituitary is not working. The 2018 European Thyroid Association (ETA) guideline on central hypothyroidism calls the TSH in that second case inappropriately low or normal: normal on paper, and too low for a body short of thyroid hormone. It also notes that central hypothyroidism is a major false negative of the reflex TSH strategy, which it describes as a widely accepted method of screening thyroid function with a first-line TSH measurement.

The last row exists because some pituitaries release TSH that the laboratory can measure but that does not work properly. The AACE and ATA guideline says people with nonfunctioning pituitary adenomas and central hypothyroidism may have mildly raised TSH, generally not above 6 or 7 mIU/L, from these less active forms. The opposite pattern, a high TSH with a normal free T4, is covered in our guide to high TSH with a normal free T4.

The ETA's Recommendation 11 confirms central hypothyroidism only after the conditions in its Table 3, which can produce the same numbers without pituitary disease, have been excluded. These are the ones that apply to adults, with what the US guidelines add.

• Illness. The 2014 ATA treatment guideline describes the nonthyroidal illness syndrome in critically ill people with normal thyroids: T3 falls first, then TSH and T4, then free T4 in some sicker patients. The AACE and ATA guideline says major illness and starvation can bring a low T3 with normal or low T4 and TSH, and that people who are not clearly hypothyroid should not be treated until the acute illness has resolved. The ETA adds that a low T3 on its own points to nonthyroidal illness or a deiodination defect, not to central hypothyroidism.
• Anti-seizure medicines. In a 1996 JAMA study of 19 people with seizure disorders, phenytoin and carbamazepine lowered free T4 on a commercial test kit while free T4 measured by ultrafiltration of undiluted serum was unchanged, and TSH stayed normal. The authors concluded that clinicians should rely on TSH to confirm thyroid status in these patients. The 2014 ATA guideline describes the same pattern, normal TSH and low free T4, as attributed to a measurement artifact.
• Medicines that lower TSH. The ETA lists glucocorticoids, dopamine, cocaine, anti-epileptics or anti-psychotics, and metformin. The AACE and ATA guideline's Table 10 adds dopamine agonists, somatostatin analogues and opiates, and says octreotide does not cause persistent central hypothyroidism while oral bexarotene almost always does.
• Pregnancy. The AACE and ATA guideline says direct free T4 immunoassays may read lower in pregnancy against non-pregnant reference ranges, and many people fall below that range in the third trimester. It recommends total T4 or a free T4 index in pregnancy. The ETA says low maternal free T4 has to be read against trimester-specific ranges.
• Recovery from an overactive thyroid, or stopping levothyroxine. Both are on the ETA's list. The AACE and ATA guideline notes that after antithyroid drugs, surgery or radioiodine, TSH may stay low for many weeks to months, so free T4 is the test that detects hypothyroidism in that window.
• Anorexia nervosa. The AACE and ATA guideline says it can produce low TSH with low free T4, mimicking central hypothyroidism.
• The assay itself. The ETA lists technical assay problems or interference, and binding-protein defects when total T4 or a free T4 index is used.

The ETA defines central hypothyroidism as reduced thyroid hormone production from an otherwise normal thyroid that is not being stimulated enough by TSH, because of a problem in the pituitary (secondary) or the hypothalamus (tertiary). It often comes with other pituitary hormone deficiencies, and in most acquired cases the hypothyroidism is mild to moderate, because the pituitary's TSH reserve is rarely completely depleted.

The causes the ETA lists for adults include pituitary macroadenomas, craniopharyngiomas and other tumors near the pituitary, cranial surgery or radiation, head trauma, pituitary infarction including Sheehan syndrome after childbirth, subarachnoid hemorrhage, lymphocytic hypophysitis, iron overload, sarcoidosis, some infections and some drugs. Its Recommendation 9 asks for central hypothyroidism to be checked in people treated with retinoid X receptor drugs such as bexarotene, with ipilimumab or other checkpoint inhibitors, or with mitotane.

Its Recommendations 6 and 7 name who should be screened: anyone with a personal or family history of hypothalamic or pituitary disease, moderate to severe head trauma, stroke, previous cranial radiation, or hemochromatosis or iron overload, and anyone with hypothyroid symptoms plus signs pointing to the pituitary, such as a high prolactin, acromegalic features, diabetes insipidus, recurrent headaches or visual field defects.

The evidence under these recommendations is thin, and the ETA says so. Its task force found only one randomized controlled trial, also drew on retrospective studies and expert opinion, and asks that the document be read as expert guidance for endocrinologists. Recommendations it rates as weak are worded as suggestions.

The ETA's Recommendation 11 asks for a free T4 below the lower limit of normal with an inappropriately low or normal TSH on at least two separate determinations, after the look-alikes are excluded. Milder forms, with a free T4 still inside the range, are harder: for people already followed for pituitary disease, Recommendation 13 suggests considering the diagnosis when free T4 falls into the lowest quarter of the normal range, particularly with a drop of more than 20% from earlier values on the same assay.

The AACE and ATA guideline's Recommendation 12 (Grade A) says that in central hypothyroidism, free T4 or a free T4 index, not TSH, should be used to diagnose and guide treatment. Its Recommendation 28 says people with other endocrine disease such as adrenal or pituitary disorders, or an unusual combination of thyroid test results, should be seen in consultation with an endocrinologist.

Once treated, the target changes too. The ETA's Recommendation 26 aims for a free T4 above the middle of the normal range, its Recommendation 27 says TSH becomes useless for monitoring when it was low at baseline, and the AACE and ATA guideline's Recommendation 24 likewise targets free T4 above the mid-normal value for the assay. The ETA's Recommendation 25 recommends avoiding treatment of milder forms, with free T4 within the lower limit of the normal range, in people older than 75.

Questions that follow from the guidelines: has the result been repeated, and on the same assay? Was I ill, in hospital, pregnant or recently treated for an overactive thyroid when it was drawn? Do I take phenytoin, carbamazepine, steroids, opioids, metformin or any cancer drug on the ETA's list? Have I had a head injury, pituitary problem, brain surgery or radiation? And have my cortisol and other pituitary hormones been checked? Our TSH reference interval page covers what a normal TSH does and does not rule out, the TSH reference page shows every band with its source and date, and the lab result lookup places a TSH value against them. Our guide to the TSH normal range explains how the interval shifts with age, and what TSH level is dangerously high sets out the warning signs of severe hypothyroidism, including the 2014 ATA guideline's point that an inappropriate TSH may be seen when hypothyroidism is caused by TSH deficiency. A low TSH with a low free T4 is covered in what TSH level is dangerously low. Your personal screening plan lists the screenings that apply at your age and sex.