Medicine Endocrinology · Medicine · High Yield

Thyroid Function Interpretation

Thyroid function tests (TFTs) — TSH and free T4 — follow a predictable pattern in each thyroid disorder. Interpreting the TSH–T4 relationship correctly is one of the most high-yield skills in general medicine.

✦ The Mnemonic

"TSH Tells The Story — Low or High, T4 Confirms"

TSH is the primary screening test; T4 confirms and quantifies

↓TSH ↑T4 Primary hyperthyroidism TSH suppressed, T4 elevated — Graves' disease, toxic nodule, toxic MNG

↓TSH →T4 Subclinical hyperthyroidism TSH suppressed, T4 normal — early or mild excess; monitor or treat

→TSH →T4 Euthyroid (normal) Both normal — no thyroid dysfunction

↑TSH →T4 Subclinical hypothyroidism TSH raised, T4 normal — early failure; treat if TSH >10 or symptomatic

↑TSH ↓T4 Primary hypothyroidism TSH elevated, T4 low — Hashimoto's, post-radioiodine, post-thyroidectomy

↓TSH ↓T4 Secondary hypothyroidism Both low — pituitary/hypothalamic failure; rare; requires MRI pituitary

📚 Clinical Breakdown

TSH (thyroid-stimulating hormone) is produced by the pituitary and operates by negative feedback — when T4 is high, TSH falls; when T4 is low, TSH rises. This makes TSH the most sensitive indicator of thyroid status. A normal TSH virtually excludes primary thyroid dysfunction. Always check TSH first; add free T4 if TSH is abnormal.

Graves' disease (autoimmune — TSH receptor antibodies stimulate the gland) is the most common cause of primary hyperthyroidism. Presents with weight loss, palpitations, heat intolerance, tremor, and exophthalmos. Treat with carbimazole (block synthesis), propranolol (symptom control), then definitive therapy: radioiodine or thyroidectomy.

Hashimoto's thyroiditis (autoimmune — TPO antibodies destroy the gland) is the most common cause of primary hypothyroidism in iodine-sufficient countries. Presents insidiously with fatigue, weight gain, cold intolerance, constipation, bradycardia, and dry skin. Treat with levothyroxine — titrate to TSH 0.5–2.5 mU/L.

First-line TFT TSH alone — most sensitive screening test

Primary hypo pattern ↑TSH + ↓T4 — most common TFT abnormality

Secondary hypo pattern ↓TSH + ↓T4 — pituitary failure; rare

Subclinical hypo treatment Treat if TSH >10 mU/L or symptomatic at lower levels

⭐ Clinical Pearl

Sick euthyroid syndrome (non-thyroidal illness): in any severe systemic illness, TFTs become abnormal without true thyroid disease — TSH may be low or high, T4 low. This is a physiological response to conserve energy. Do not initiate thyroid treatment based on TFTs taken during acute illness — recheck 6 weeks after recovery.

⚠ Exam Trap

Amiodarone and thyroid function: amiodarone (40% iodine by weight) causes both hypo- and hyperthyroidism. It also interferes with T4→T3 conversion, making TFT interpretation complex. Always check TFTs before starting amiodarone and every 6 months during treatment. Amiodarone-induced thyrotoxicosis (AIT) can be difficult to treat and may require steroids or thyroidectomy.

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