TSH Interpreter Thyroid Function Test

1Thyroid Function Pattern Recognition

Levothyroxine (T4) Replacement Dosing

• Young healthy adults (overt hypothyroidism): Full replacement dose — 1.6 µg/kg/day. Start at full dose in healthy adults under 60
• Elderly or cardiac disease: Start low — 25–50 µg/day, increase by 25 µg every 4–8 weeks. Avoid precipitating angina or AF
• Subclinical hypothyroidism (TSH 4–10, normal FT4): 50 µg/day starting dose if treating. Repeat TFT in 6–8 weeks. Target TSH 0.5–2.5 mIU/L for most adults; 0.5–1.5 in younger patients; can accept 1–4 in elderly
• Pregnancy: Increase levothyroxine dose immediately by 25–30% on diagnosis of pregnancy. Target TSH <2.5 mIU/L in first trimester, <3 mIU/L in second and third trimester
• Monitoring: Recheck TFT 6–8 weeks after any dose change. Once stable, annual TSH monitoring

Causes of Hypothyroidism

• Hashimoto thyroiditis (autoimmune): Most common cause in iodine-sufficient countries. Anti-TPO antibodies positive in 95%. Gradual destruction of thyroid tissue
• Iodine deficiency: Most common cause globally. Endemic in inland mountainous regions and certain parts of South Asia and Africa
• Post-radioactive iodine (RAI) or thyroidectomy
• Drug-induced: Amiodarone (iodine content), lithium (inhibits thyroid hormone release), interferon-α, checkpoint inhibitors
• Central (secondary/tertiary): Pituitary or hypothalamic disease — low TSH with low FT4

Causes of Hyperthyroidism

• Graves disease: Most common cause. Autoimmune stimulation of TSH receptor by TSH-receptor antibodies (TRAb). Diffuse goitre, ophthalmopathy, pretibial myxoedema
• Toxic nodular goitre (Plummer disease): Single or multiple autonomously functioning nodules — common in iodine-deficient areas and elderly
• Subacute (de Quervain) thyroiditis: Post-viral painful thyroiditis with transient hyperthyroid phase, followed by hypothyroid phase, then recovery. Low uptake on thyroid scan
• Exogenous thyroid hormone (factitious): Low thyroglobulin level distinguishes from intrinsic overproduction
• Amiodarone-induced thyrotoxicosis (AIT): Type 1 (excess iodine) or Type 2 (destructive thyroiditis)

2About the TSH Interpreter

Thyroid disorders are among the most common endocrine conditions in India, with hypothyroidism (particularly Hashimoto thyroiditis) affecting an estimated 10-12% of the adult population. TSH (thyroid stimulating hormone) is the pituitary hormone that regulates thyroid function and is the single most sensitive test for thyroid dysfunction — it rises before T4 falls (in early hypothyroidism) and falls before T4 rises (in early hyperthyroidism), making it the ideal screening and monitoring test.

TSH interpretation requires clinical context. TSH can be transiently suppressed in acute illness (non-thyroidal illness syndrome / sick euthyroid syndrome) without true hyperthyroidism. TSH can be falsely elevated in the recovery phase of illness, by certain drugs (lithium, amiodarone, metoclopramide), and by assay interference from heterophile antibodies. A mildly abnormal TSH in a hospitalised acutely ill patient should be repeated after recovery before committing to thyroid diagnosis.

Amiodarone deserves special mention: it contains 37% iodine by weight and causes complex thyroid effects. It inhibits T4-to-T3 conversion, causing elevated T4 and TSH in early use (euthyroid state). Amiodarone-induced hypothyroidism (AIH) requires levothyroxine treatment. Amiodarone-induced thyrotoxicosis (AIT) is more complex — Type 1 (excess iodine, pre-existing thyroid disease) and Type 2 (destructive thyroiditis) require different treatment (carbimazole vs prednisolone respectively).

Medical disclaimer: This calculator is for educational and clinical decision-support purposes only. It does not replace clinical judgment or specialist consultation. RxMedCalc is not liable for clinical decisions made solely on this tool.