Thyroid first-line pair
TSH and Free T4: how to read them together
TSH and free T4 are the first-line thyroid axis pair: TSH reflects the pituitary signal and free T4 reflects what the thyroid actually delivers. Together they distinguish primary thyroid disease from secondary pituitary disease.
Quick answer
TSH and Free T4 are interpreted together because tSH and free T4 are the first-line thyroid axis pair: TSH reflects the pituitary signal and free T4 reflects what the thyroid actually delivers. Together they distinguish primary thyroid disease from secondary pituitary disease. UK reference: NICE CKS Hypothyroidism / NICE CKS Hyperthyroidism / NICE NG145.
Why these markers are ordered together
The thyroid axis is a negative-feedback loop: the pituitary releases TSH, which tells the thyroid to make T4. As T4 rises, it suppresses TSH. In primary thyroid disease (the common case), the thyroid itself is the problem, so TSH moves in the opposite direction to T4, high TSH with low T4 means hypothyroidism, low TSH with high T4 means hyperthyroidism. When TSH and T4 move in the same direction, the pituitary is the problem (rare but important, central hypothyroidism, pituitary tumour, or sick euthyroid syndrome).
Four scenarios, four interpretations
The 2x2 interpretation matrix our clinicians use when these markers come back together.
High TSH + Low free T4
What it likely means: Primary hypothyroidism. The thyroid is failing to produce enough hormone and the pituitary is driving TSH up in compensation.
What to do next: Add TPO antibodies to identify autoimmune (Hashimoto thyroiditis) as the cause (the commonest UK aetiology). Start levothyroxine per NICE CKS Hypothyroidism. Repeat TSH 6-8 weeks after starting or dose-adjusting.
Low TSH + High free T4
What it likely means: Primary hyperthyroidism. The thyroid is producing too much hormone and the pituitary is suppressing TSH.
What to do next: Add TSH receptor antibodies (TRAb) for Graves disease confirmation. Free T3 to characterise (T3-predominant in some Graves cases). Endocrinology referral per NICE CKS Hyperthyroidism.
High TSH + Normal free T4
What it likely means: Subclinical hypothyroidism. Often asymptomatic, sometimes with mild symptoms.
What to do next: TPO antibodies to identify autoimmune predisposition. Annual TSH monitoring per NICE CKS, risk of progression to overt hypothyroidism is approximately 2-4 percent per year if TPO-positive. Treatment decisions are individualised.
Low TSH + Low free T4
What it likely means: Central (pituitary) hypothyroidism, rare but important. Or sick euthyroid syndrome in acute illness.
What to do next: Pituitary work-up: morning cortisol, prolactin, FSH/LH, IGF-1. MRI pituitary if biochemistry suggests central cause. Endocrinology referral.
When the pair is ambiguous
In acute illness, sick euthyroid syndrome can produce a low free T4 with normal or low TSH. Repeat thyroid function 4-6 weeks after recovery from any significant illness. Biotin supplements at high dose interfere with both TSH and free T4 immunoassays, hold for 48-72 hours before testing.
Individual markers
Read about each marker in isolation:
Get the pair tested together
Both markers in one blood draw at 134 Harley Street. Custom panels designed by a GMC-registered doctor from £275, or fixed panels including this pair from £249.