Thyroid Function Test Interpretation & Nonthyroidal Illness
When the TSH lies: pituitary disease, sick patients, binding proteins, and biotin
Reading thyroid function tests
Why TSH comes first. Pituitary TSH responds to free T4 and T3 in a log-linear way, so a 2-fold change in free T4 shifts TSH roughly 100-fold. With an intact hypothalamic–pituitary axis and stable thyroid status, TSH is therefore the most sensitive screening test, and free T4 is added when TSH is abnormal or pituitary disease is possible.
When TSH misleads. TSH is unreliable in hypothalamic or pituitary disease, TSH-secreting tumors, resistance to thyroid hormone, acute illness, the weeks after a dose change or treatment of hyperthyroidism, early pregnancy, with certain drugs, and with assay interference.
| TSH | Free T4 | Usual interpretation |
|---|---|---|
| ↑ | ↓ | Primary hypothyroidism |
| ↑ | Normal | Subclinical hypothyroidism; recovery from illness; assay artifact |
| ↓ | ↑ (or only T3 ↑) | Primary hyperthyroidism (T3 toxicosis if only T3 is high) |
| ↓ | Normal, with normal T3 | Subclinical hyperthyroidism; illness, drugs, first trimester |
| ↓ or normal | ↓ | Central hypothyroidism or nonthyroidal illness |
| Normal or ↑ | ↑ | TSH-secreting adenoma, hormone resistance or assay interference |
Central hypothyroidism
Diagnosis. A low free T4 with a low or inappropriately normal TSH (occasionally mildly elevated but biologically inactive) indicates pituitary or hypothalamic failure. Causes include pituitary macroadenomas, pituitary surgery or radiation, Sheehan syndrome, apoplexy, hypophysitis (including immune checkpoint inhibitor therapy) and craniopharyngioma. Evaluation includes pituitary MRI and testing of the other axes: morning cortisol, prolactin, LH and FSH with testosterone or estradiol, and IGF-1.
Treatment. Adrenal insufficiency is excluded or treated first, because levothyroxine accelerates cortisol clearance and can precipitate adrenal crisis, so hydrocortisone is started before thyroid hormone. Levothyroxine is titrated to a free T4 in the upper half of the reference range; TSH stays low and is ignored.
A low free T4 with a normal TSH is never reassuring: once nonthyroidal illness is excluded, it is the signature of central hypothyroidism.
High free T4 with a nonsuppressed TSH
Approach. Elevated free T4 and T3 with a normal or high TSH means inappropriate TSH secretion. First exclude laboratory artifact (biotin, heterophile antibodies) by stopping supplements and repeating on a different platform. A genuine pattern reflects a TSH-secreting pituitary adenoma or resistance to thyroid hormone.
| Feature | TSH-secreting adenoma | Thyroid hormone resistance |
|---|---|---|
| Genetics | Sporadic | Usually autosomal dominant THRB mutation; relatives affected |
| Clinical | Hyperthyroid; headache, bitemporal hemianopia | Variable: goiter, tachycardia, often few symptoms |
| α-subunit | High (α-subunit:TSH ratio >1) | Normal |
| Pituitary MRI | Adenoma, often macroadenoma | Normal |
| Treatment | Transsphenoidal surgery; somatostatin analog as bridge | Usually none; β-blocker for symptoms |
Radioiodine or antithyroid drugs alone are wrong for both: removing feedback raises TSH further and can enlarge a thyrotropinoma or goiter.
Nonthyroidal illness, binding proteins and pregnancy
Nonthyroidal illness. In sepsis, surgery, myocardial infarction, starvation and other acute illness, peripheral T4-to-T3 conversion falls and T4 is diverted to reverse T3, giving low T3, high reverse T3 and a normal or low TSH. Prolonged critical illness also lowers free T4, which predicts mortality. Dopamine and high-dose glucocorticoids suppress TSH further. The pattern is adaptive and thyroid hormone does not help. During recovery TSH can rise transiently, usually below 20 mIU/L, mimicking primary hypothyroidism. Testing is avoided in acutely ill patients unless dysfunction is strongly suspected and is repeated 4–6 weeks after recovery. Known pituitary disease, other hormone deficits or a free T4 that stays low suggest true central hypothyroidism.
Binding proteins. About 99.97% of T4 is protein-bound, mostly to thyroxine-binding globulin (TBG), so TBG changes alter total T4 and T3 without changing free hormone or TSH. Estrogen (pregnancy, oral contraceptives, oral HRT, tamoxifen) and acute hepatitis raise TBG; androgens, high-dose glucocorticoids, nephrotic syndrome and cirrhosis lower it. These patients are euthyroid and need no treatment.
Pregnancy. hCG weakly stimulates the TSH receptor, so TSH is lower in the first trimester and may be suppressed with hyperemesis, while estrogen-driven TBG raises total T4 about 1.5-fold; trimester-specific TSH ranges are used. Gestational transient thyrotoxicosis resolves by the second trimester without antithyroid drugs, whereas Graves disease has TSH-receptor antibodies. Hypothyroid women need about 25–30% more levothyroxine as soon as pregnancy is confirmed.
Subclinical thyroid disease
Subclinical hypothyroidism. An elevated TSH with a normal free T4 is confirmed by repeating both in 6–12 weeks, since many values normalize; TPO antibodies predict progression. Levothyroxine is indicated for TSH ≥10 mIU/L and for women who are pregnant or trying to conceive. Between the upper limit and 10, treatment is considered in younger adults with symptoms or positive antibodies, but not in older adults, whose TSH normally drifts upward and who gain no benefit.
Subclinical hyperthyroidism. A low TSH with normal free T4 qualifies only after T3 is also normal, because a high T3 alone is T3 toxicosis. Repeat in 6–12 weeks and exclude levothyroxine over-replacement (managed by lowering the dose), nonthyroidal illness, glucocorticoids and early pregnancy. Persistent endogenous disease, often toxic multinodular goiter or a toxic adenoma in older adults (identified by radioiodine scan; hot nodules are rarely malignant and need no biopsy), is treated, usually with radioiodine, when TSH is below 0.1 and the patient is ≥65 or has atrial fibrillation or heart disease, osteoporosis or symptoms, because it raises atrial fibrillation risk and accelerates bone loss.
Assay and drug pitfalls
Biotin. High-dose biotin, taken for hair, skin and nails or multiple sclerosis, disrupts streptavidin-biotin immunoassays: sandwich assays such as TSH read falsely low, and competitive assays such as free T4, T3 and TSH-receptor antibodies read falsely high, creating a Graves-like pattern in a euthyroid patient. Stopping biotin for at least 2 days and repeating the tests resolves it.
Interfering antibodies. Heterophile antibodies, such as human anti-mouse antibodies, and macro-TSH can yield a spuriously high TSH. Results that do not fit the clinical picture are repeated on another platform or with dilution or blocking studies before treatment. Heparin can falsely raise measured free T4 by generating free fatty acids in the sample.
Drugs and timing. Dopamine, high-dose glucocorticoids and somatostatin analogs suppress TSH. TSH lags behind changes in thyroid hormone, so it is rechecked 6–8 weeks after a levothyroxine dose change; after treatment of hyperthyroidism TSH may stay suppressed for months, and free T4 and T3 guide therapy. Blood is drawn before the daily levothyroxine dose because free T4 rises for several hours after ingestion.