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A Low Free T4 with a TSH That Is Not Raised

By the Prismo editorial team Medically reviewed by Dr. William Parker Reviewed 24 August 2026 4 min read

When thyroid hormone falls, the pituitary is supposed to shout. A TSH sitting comfortably in the middle of its range while the free T4 is below its own is not a normal answer, it is an absent one, and recognizing a normal number as the wrong number is the whole of this pattern.

The pattern on your report

  • TSH Normal Key
  • Free T4 Low · moderate Key
  • Free T3 Low-normal Supporting

Printed as: Free T3 in pmol/Lor pg/mLFree T4 in pmol/Lor ng/dLTSH in mIU/Lor uIU/mL— The same number under two names. In this pattern its value matters less than the fact that it has not risen.

Why the numbers look like this

The feedback loop has two halves. The thyroid makes hormone; the pituitary measures it and adjusts TSH. Everything on the other thyroid pages assumes the pituitary half is working, and the TSH is trusted because of that assumption.

When the pituitary or the hypothalamus above it is the damaged part, the assumption fails. Thyroid hormone falls and no instruction is sent, so the TSH stays unremarkable while the T4 drifts down. Worse, the TSH that is produced can be biologically weak while still being detected by the assay, which is why the number can even look mid-range.

So this pattern is not a thyroid problem at all. It is a pituitary problem presenting on a thyroid panel, and the thyroid is doing exactly what it is being told.

Not being flagged is not the same as normal

There is no flag for inappropriateness. Both numbers can print without a mark beside them, or only the T4 flags, and the report has no way to say that the pair does not make sense together. That is why this is missed: nothing on the page is obviously wrong. Assay interference produces the same picture, so a result that does not fit the person is worth repeating on a different platform before a pituitary work-up begins.

What else on the report can hide this

If the pituitary is failing at one hormone it is often failing at others, and two of those are more urgent than the thyroid. A morning cortisol is the priority, because unrecognized adrenal insufficiency is dangerous and, critically, giving levothyroxine before steroid replacement in someone with both can precipitate a crisis. Thyroid hormone first is the wrong order.

The rest of the pituitary panel follows: LH, FSH, testosterone or estradiol, prolactin and IGF-1. A raised prolactin alongside suggests a mass pressing on the stalk.

Separately, non-thyroidal illness produces a similar picture during serious illness and recovers on its own, so the setting in which the sample was taken matters as much as the numbers.

What usually causes it

Listed from most to least common — not from most to least serious.

  1. Very common

    Non-thyroidal illness

    Any serious illness alters thyroid tests without thyroid disease. The commonest explanation in a hospital setting, and it recovers without treatment.

  2. Common

    Central hypothyroidism from a pituitary problem

    A pituitary tumor, previous surgery or radiotherapy, or damage from bleeding or injury. Other pituitary hormones are usually affected too, which is what to check next.

  3. Common

    Medications

    High-dose steroids, dopamine, opioids and some anticonvulsants suppress TSH centrally. Timing against the drug identifies it.

  4. Common

    Assay interference

    Biotin, heterophile antibodies or binding-protein abnormalities. Repeating on a different platform is the cheapest way to exclude it.

  5. Uncommon

    Recovery from thyroiditis or recent overactivity

    TSH can stay suppressed for weeks after the thyroid hormone level has fallen, producing this pattern transiently on the way back to normal.

  6. Uncommon

    Pituitary damage after childbirth — in after a delivery with heavy blood loss

    Sheehan syndrome. Failure to lactate and absent periods after a difficult delivery are the historical clues, sometimes years earlier.

  7. Rare

    Infiltrative pituitary disease

    Sarcoidosis, hemochromatosis or hypophysitis, including after immune checkpoint inhibitor treatment. Increasingly recognized as those drugs are used more.

What is usually checked next

  • Morning cortisol The priority. Adrenal insufficiency must be identified before thyroid hormone is replaced, because the wrong order can precipitate a crisis.
  • The rest of the pituitary panel LH, FSH, testosterone or estradiol, prolactin and IGF-1 establish how much of the pituitary is affected.
  • Repeat on a different assay platform Excludes interference, which produces this picture with a healthy pituitary and thyroid.
  • MRI of the pituitary Images the gland once the biochemistry supports a central cause.
  • Repeat when well, if the sample was taken during illness Non-thyroidal illness is the commonest explanation and it resolves on its own.

When to seek care sooner

  • Emergency Dizziness on standing, vomiting, or collapse
  • Emergency Severe headache with visual disturbance or double vision
  • Emergency Confusion or drowsiness
  • Same day Low blood pressure with a low sodium
  • Same day Starting levothyroxine before adrenal function has been checked
  • Soon Loss of periods, loss of body hair, or loss of libido alongside

Questions worth bringing to your appointment

  1. My TSH is normal but my T4 is low. Is that combination expected?
  2. Has a morning cortisol been checked before any thyroid treatment starts?
  3. Should the rest of my pituitary hormones be tested?
  4. Was I unwell when this sample was taken?
  5. Could this be assay interference, and can it be repeated on a different platform?

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