🔬 Thyroid Function Tests

Thyroid Function Tests (TFTs) are used to evaluate the activity of the thyroid gland. The primary markers measured are Thyroid Stimulating Hormone (TSH) and Free Thyroxine (fT4). These tests help diagnose hyperthyroidism and hypothyroidism, monitor treatment with thyroid hormones or antithyroid medications, and investigate non-specific symptoms such as fatigue or palpitations. TSH is released by the pituitary to regulate the production of T4 and T3 by the thyroid gland.

Indicated for symptoms of hypothyroidism (weight gain, cold intolerance, bradycardia, depression, constipation) or hyperthyroidism (weight loss, heat intolerance, tachycardia, anxiety, tremor). Used in the investigation of atrial fibrillation, unexplained weight changes, menstrual irregularities, and infertility. TFTs are also used for monitoring patients on thyroid-altering medications and screening newborns (neonatal heel prick) for congenital hypothyroidism.

A venous blood sample is collected into a serum separator tube (SST, yellow top). The lab uses automated chemiluminescent immunoassays to measure TSH and free thyroxine (fT4). Free triiodothyronine (fT3) is usually only added by the lab if hyperthyroidism is suspected (low TSH). The 'reflex testing' protocol often starts with TSH alone and only proceeds to T4/T3 if TSH is outside the reference range to improve cost-efficiency.

Normal results consist of TSH and fT4 within the laboratory's reference ranges. For most UK labs, TSH is approximately 0.4–4.5 mU/L and free T4 is 12–22 pmol/L. A normal TSH generally excludes primary thyroid dysfunction. In a patient on levothyroxine, the goal is usually a TSH in the lower half of the reference range (0.4–2.0 mU/L), though this is personalised.

TSH is the most sensitive screen; if TSH is normal, significant thyroid dysfunction is unlikely. If TSH is abnormal, free T4 (and sometimes T3) is measured to confirm the diagnosis. In 'Sick Euthyroid Syndrome' (non-thyroidal illness), TFTs may be deranged (low TSH, low T4) during acute systemic illness; these should be repeated after recovery. Isolated low TSH can occur in the first trimester of pregnancy due to hCG's structural similarity to TSH. Subclinical disease management often depends on the degree of TSH elevation and the presence of symptoms or antibodies.

Hyperthyroidism (Primary) is characterised by a suppressed TSH (<0.01 mU/L) and elevated T4/T3; most commonly caused by Graves' disease (look for TSH-receptor antibodies). Hypothyroidism (Primary) presents with an elevated TSH and low T4; Hashimoto’s thyroiditis is the leading cause (look for TPO antibodies). Subclinical hypothyroidism features a mildly raised TSH with normal T4. Secondary (central) hypothyroidism is marked by a low TSH and a low T4, suggesting pituitary or hypothalamic pathology. Extremely high T4 with high TSH may indicate a rare TSH-secreting pituitary adenoma or thyroid hormone resistance.

TFTs are essential for diagnosing conditions that drastically affect metabolic rate, cardiovascular health, and bone density. In pregnancy, thyroid health is critical for foetal neurodevelopment, and reference ranges change (lower TSH is normal in the first trimester). TFTs are used to monitor the efficacy of levothyroxine replacement or antithyroid drug therapy (carbimazole). Many medications, such as amiodarone and lithium, can induce thyroid dysfunction, necessitating baseline and periodic TFT monitoring. They are also part of the standard 'confusion screen' in the elderly.

Testing too soon (before 6-8 weeks) after starting or changing a levothyroxine dose. Misinterpreting Sick Euthyroid Syndrome as true hypothyroidism and starting unnecessary thyroxine. Failing to check a T3 level in a patient with a suppressed TSH but normal T4 (missing T3-toxicosis). Ignoring the possibility of pituitary disease when both TSH and T4 are low. Not accounting for the physiological changes in TSH during the first trimester of pregnancy.

TFTs can be misleading during acute systemic illness (Sick Euthyroid). They may take 6-8 weeks to stabilize after a change in levothyroxine dose, so premature testing is not useful. Standard tests measure 'Free' hormones, but factors affecting binding proteins (e.g., the oral contraceptive pill or nephrotic syndrome) can sometimes complicate interpretation of total hormone levels. TFTs do not provide information on the structural nature of the thyroid (e.g., nodules), which requires ultrasound.抽血时间一般没有特殊要求，但服药后的峰值可能会短暂影响结果。

Be comfortable with the feedback loop: High T4 inhibits TSH; low T4 stimulates TSH. Understand that in primary disease, TSH and T4 move in opposite directions, while in secondary (pituitary) disease, they move in the same direction. Learn the specific management of subclinical hypothyroidism (treat if TSH >10 or TPO positive with symptoms). Recognise that amiodarone can cause both hyper- and hypothyroidism.