A Low TSH with a Low Free T4
Both low together points at the pituitary, not the thyroid. A failing thyroid drives TSH up as the gland is asked to work harder; here nothing is doing the asking, so the gland is quiet because the instruction never arrived. That changes the assessment from the ground up, and one part of it is genuinely urgent: cortisol must be checked and replaced before any thyroid treatment starts.
The pattern on your report
- TSH Low Key
- Free T4 Low Key
- Morning cortisol Low Key
- Sodium Low-normal Key
Printed as: Morning cortisol in nmol/Lor ug/dL— Strong daily rhythm, so a morning sample is required. This one is checked before thyroid treatment, not after.Free T4 in pmol/Lor ng/dL— The value treatment is followed on here, which is the reverse of the usual arrangement.Sodium in mmol/Lor mEq/L— Often low when cortisol is deficient, which is a supporting clue to the wider problem.TSH in mIU/Lor uIU/mL— Cannot be used to judge the dose in this situation, because the pituitary that produces it is the part not working.
Why the numbers look like this
The pituitary senses circulating thyroid hormone and releases TSH to keep it steady. When the thyroid itself fails, hormone levels fall, the pituitary notices, and TSH rises. That rise is the signature of the common form of an underactive thyroid.
When the pituitary is the problem, that feedback cannot operate. TSH stays low or unremarkable despite a low hormone level, and the thyroid, receiving no instruction, produces less. The gland is normal and idle.
The pituitary rarely fails one hormone at a time, which is why the other axes matter. Cortisol is the one that cannot wait, because thyroid hormone increases the body's demand for it. Starting levothyroxine in someone whose cortisol is already low raises that demand beyond what the adrenal glands can supply, and can precipitate a collapse.
One further consequence follows for treatment. Because TSH cannot respond normally, it cannot be used to judge whether the dose is right. Free T4 is followed instead, reversing the usual arrangement and confusing a great many people.
Not being flagged is not the same as normal
The same combination appears during and after serious illness, when the axis is suppressed from the top down, and that is far commoner than pituitary disease. Distinguishing them relies on timing and on the other pituitary hormones rather than on the thyroid numbers themselves. Methods for measuring free T4 vary more between laboratories than TSH methods do, so a value at the lower end in one place can sit comfortably in range at another.
What else on the report can hide this
A morning cortisol comes first, and it comes before treatment, not alongside it. This step changes safety, not the diagnosis.
The rest of the pituitary follows: prolactin, the gonadal hormones, and growth hormone assessment where indicated. A pattern of several deficiencies is what confirms the level of the problem.
Imaging of the pituitary is arranged once the biochemistry supports it, and it identifies tumors, previous damage and infiltrative conditions.
The history often points directly. A difficult delivery with major blood loss, head injury, cranial radiotherapy, pituitary surgery, or long-term opioid use each affect this axis, and opioids in particular are common and rarely considered.
Non-thyroidal illness has to be excluded before any of this, because it produces the same numbers and is far commoner. A repeat once someone has fully recovered resolves most of these without a single pituitary test.
What usually causes it
Listed from most to least common — not from most to least serious.
- Very common
Non-thyroidal illness
Suppresses the axis and produces these numbers without any pituitary disease. Far commoner than the alternatives, and a repeat after recovery settles it.
- Very common
Recent steroid treatment
Suppresses TSH directly. Includes courses recently finished, and inhaled or injected forms that people do not count.
- Common
A pituitary tumor
Usually with other hormone deficiencies, and sometimes headache or visual field loss. Imaging identifies it.
- Common
Long-term opioid treatment
Suppresses this axis and the gonadal one. Common, reversible, and rarely considered as an explanation.
- Common
Previous pituitary surgery or radiotherapy
From the history, sometimes many years earlier. Deficiencies can appear long after the treatment.
- Common
Over-treatment with levothyroxine
Not this pattern strictly, since free T4 is usually high, but a recently reduced dose can produce a transient version of it.
- Uncommon
Sheehan syndrome — in people who had major blood loss at delivery
Pituitary damage from that event, often with failure to lactate at the time and deficiencies appearing later.
- Uncommon
Infiltrative or inflammatory pituitary disease
Sarcoidosis, hemochromatosis and hypophysitis among others. Suspected when several axes fail together.
- Uncommon
Traumatic brain injury
Pituitary deficiency is an under-recognized consequence, sometimes emerging months afterwards.
What is usually checked next
- A morning cortisol, before any thyroid treatment starts Thyroid hormone raises the demand for cortisol, so replacing it first can precipitate a collapse. This is a safety step rather than a diagnostic one.
- Repeat after full recovery from any illness Non-thyroidal illness produces the identical pattern and is far commoner than pituitary disease.
- The remaining pituitary hormones Several deficiencies together confirm the level of the problem and direct what needs replacing.
- A drug history including steroids and opioids Both suppress this axis, both are common, and both are reversible.
- Pituitary imaging once the biochemistry supports it Identifies tumors, previous damage and infiltrative disease.
When to seek care sooner
- Emergency Collapse, severe weakness, or confusion during an illness
- Emergency Severe headache with visual disturbance
- Emergency Vomiting with dizziness on standing
- Emergency Marked drowsiness with a low body temperature
- Same day Loss of peripheral vision
- Soon Starting levothyroxine before cortisol has been checked
Questions worth bringing to your appointment
- Has my cortisol been checked before starting thyroid treatment?
- Was I unwell around the time of this blood test?
- Have the other pituitary hormones been measured?
- Am I on opioids or steroids that could explain this?
- Will my dose be followed on free T4 rather than TSH?