A Low TSH with a Raised Free T4
This is an overactive thyroid, and unlike most patterns on these pages the diagnosis is not the hard part. What matters next is the cause, because the three common ones are treated in completely different ways, and one of them needs no treatment at all beyond waiting.
The pattern on your report
- TSH Low · marked Key
- Free T4 High · moderate Key
- Free T3 High Key
Printed as: Free T3 in pmol/Lor pg/mLFree T4 in pmol/Lor ng/dLTSH in mIU/Lor uIU/mL
Why the numbers look like this
Thyroid hormone in excess speeds nearly everything: heart rate, gut transit, heat production, bone turnover. The pituitary responds by shutting TSH down as far as it can, which is why the TSH is suppressed rather than merely low.
Where the excess comes from is the question the numbers cannot answer. A gland being driven to overproduce, as in Graves disease, is making new hormone continuously and will keep doing so. An inflamed gland leaking its stored hormone, as in thyroiditis, has a finite supply and runs out. A single autonomous nodule produces independently of any signal.
Those three behave differently over time and respond to different treatments, and the drugs that block hormone production do nothing at all for the leaking kind, because nothing is being produced.
Not being flagged is not the same as normal
The height of the free T4 tracks severity loosely but not reliably, and free T3 is often raised proportionally more, so a T4 that looks only modestly high can accompany substantial symptoms. Two situations distort the reading. Amiodarone produces its own complicated patterns. And high-dose biotin interferes with the assays in a direction that mimics this exact result, so a low TSH with a high free T4 in someone taking hair and nail supplements deserves a repeat off the supplement before it is believed.
What else on the report can hide this
TRAb is the test that splits the differential. Positive means Graves disease, which is autoimmune, persistent, and treated with blocking drugs, radioiodine or surgery. Negative moves the question to a nodule or to thyroiditis, and an uptake scan separates those two: high uptake means the gland is working hard, near-zero uptake means it is leaking.
That distinction changes the treatment entirely, which is the practical reason it comes first.
Check a full blood count and liver enzymes before any antithyroid drug is started, because carbimazole and propylthiouracil can both cause a sudden severe fall in neutrophils and both can affect the liver. Knowing the baseline is what makes a later result interpretable, and a sore throat or fever on those drugs is an instruction to have a count that day.
What usually causes it
Listed from most to least common — not from most to least serious.
- Very common
Graves disease
The commonest cause in younger adults. TRAb positive, a diffusely enlarged gland, and sometimes eye changes. It persists and needs active treatment.
- Very common
Toxic nodular goiter or a single toxic nodule — in more frequent with age
An autonomous area producing regardless of TSH. Uptake is focal on scanning, and definitive treatment is usually radioiodine or surgery rather than long-term tablets.
- Common
Thyroiditis
Stored hormone leaking from an inflamed gland after a virus, after childbirth, or painlessly. Uptake is near zero, it burns out over weeks, and antithyroid drugs do nothing because nothing is being made.
- Common
Too much thyroid hormone taken
Prescribed, taken deliberately for weight loss, or present undeclared in a supplement. Thyroglobulin is low, which distinguishes it from every glandular cause.
- Uncommon
Amiodarone
Iodine-rich, and it causes overactivity by two different mechanisms needing different treatments. Complicated enough that it is managed with specialist input.
- Uncommon
Excess iodine
Contrast dye, kelp supplements or iodine-rich preparations, particularly where a nodular goiter already exists.
- Uncommon
Assay interference from high-dose biotin
Produces exactly this pattern with a healthy thyroid. Stopping it for a couple of days and repeating settles it, and it is worth excluding before treatment starts.
- Rare
A TSH-secreting pituitary tumor
Gives a raised T4 with a TSH that is not suppressed, which is a different pattern and the reason the TSH direction is checked rather than assumed.
What is usually checked next
- TRAb antibodies Splits Graves disease from everything else, and it is the fastest route to a treatment decision.
- Uptake scan Distinguishes a gland working hard from one leaking, which determines whether antithyroid drugs will help at all.
- Full blood count and liver enzymes before starting treatment Establishes the baseline that makes later monitoring meaningful, since both antithyroid drugs can affect neutrophils and the liver.
- A supplement review, with a repeat off biotin Excludes an assay artifact that mimics this result precisely.
- Thyroglobulin Low where the hormone is being taken rather than made, which is otherwise difficult to establish.
When to seek care sooner
- Emergency Fever with agitation, confusion, vomiting and a racing pulse
- Emergency An irregular heartbeat, chest pain, or breathlessness at rest
- Emergency Sore throat, mouth ulcers or fever while taking an antithyroid drug
- Emergency Yellowing of the eyes while taking an antithyroid drug
- Same day New bulging eyes, double vision, or pain and redness in an eye
- Soon Pregnancy, or planning one, with an overactive thyroid
Questions worth bringing to your appointment
- Has TRAb been checked, and do I need an uptake scan?
- Is this Graves disease, a nodule, or thyroiditis, and how does that change treatment?
- Were my blood count and liver enzymes checked before starting the tablets?
- What symptoms mean I should have a blood test the same day?
- Could a supplement I take have affected the result?