1Thyroid Function Test Pattern Recognition
| TSH Level | Free T4 | Free T3 | Clinical Interpretation |
|---|---|---|---|
| Normal (0.4–4.0) | Normal | Normal | Euthyroid — Normal thyroid function. |
| High (> 4.0) | Low (< 0.8) | Low / Normal | Overt Hypothyroidism — initiate levothyroxine. |
| High (4.1–10.0) | Normal | Normal | Subclinical Hypothyroidism — consider treatment if TSH >10 or antibodies positive. |
| Very High (> 10.0) | Normal or Low | — | Significant Hypothyroidism — treatment indicated. |
| Low (< 0.4) | High (> 1.8) | High (> 4.2) | Overt Hyperthyroidism — antithyroid drugs / radioiodine. |
| Low (< 0.4) | Normal | Normal | Subclinical Hyperthyroidism — monitor or treat if symptomatic / elderly. |
| Low (< 0.4) | Low | Low | Central Hypothyroidism — pituitary or hypothalamic etiology. |
| Low or Normal | Low | Very Low | Sick Euthyroid Syndrome — non-thyroidal systemic illness. |
2Levothyroxine (T4) Replacement Dosing Protocols
- Young Healthy Adults (Overt Hypothyroidism): Initiate full replacement dose at 1.6 µg/kg/day actual body weight.
- Elderly (>65 years) or Cardiac Disease: Start low at 12.5 to 25 µg/day, titrating upward slowly by 12.5–25 µg every 6 to 8 weeks to prevent precipitating angina or atrial fibrillation.
- Pregnancy (Pre-existing Hypothyroidism): Increase levothyroxine dose immediately by 25% to 30% upon confirming pregnancy (often by taking two extra tablets twice weekly) to meet higher fetal demands. Target TSH <2.5 mIU/L in the first trimester.
3Etiology of Thyroid Disorders
Causes of Hypothyroidism
- Hashimoto Thyroiditis: Autoimmune destruction (Anti-TPO positive in ~95%). Most common cause in India.
- Iatrogenic: Post-thyroidectomy or radioactive iodine (RAI) ablation.
- Drug-Induced: Amiodarone, lithium, tyrosine kinase inhibitors, immune checkpoint inhibitors.
Causes of Hyperthyroidism
- Graves Disease: Autoimmune receptor stimulation (TRAb positive). Diffuse goiter, ophthalmopathy.
- Toxic Multinodular Goiter: Autonomously functioning follicular adenomas.
- Subacute Thyroiditis: Post-viral painful tender thyroid with triphasic clinical course.
4Frequently Asked Questions
What is subclinical hypothyroidism and when should it be treated?+
Subclinical hypothyroidism is characterized by an elevated TSH (typically 4.0–10.0 mIU/L) with a completely normal Free T4. Treatment is indicated if TSH exceeds 10 mIU/L, if anti-TPO antibodies are positive, if symptoms of hypothyroidism are troublesome, or if the patient is pregnant or planning conception.
How should levothyroxine be taken for maximum absorption?+
Levothyroxine must be taken on an empty stomach with a full glass of water, 30 to 60 minutes before breakfast (or at bedtime at least 4 hours after the last meal). Calcium supplements, iron tablets, antacids, and proton pump inhibitors impair absorption and should be spaced at least 4 hours apart from thyroid medication.
What is Sick Euthyroid Syndrome (Non-Thyroidal Illness)?+
Sick Euthyroid Syndrome occurs in hospitalized or critically ill patients without primary thyroid pathology. It features low Free T3, variable Free T4, and normal or low TSH. This represents a normal metabolic adaptation to acute illness; giving thyroid hormone replacement is neither necessary nor beneficial.
Clinical Takeaway: Always interpret TSH in conjunction with Free T4. Recheck thyroid function 6 to 8 weeks after any adjustment in levothyroxine dosage.
Medical Disclaimer: This clinical interpreter provides educational decision-support. Laboratory assays vary by institution; always verify local reference ranges and correlate biochemistries with full clinical examination.