Hypothyroidism
Underactive thyroid — the gland produces too little hormone. The leading global cause in iodine-replete populations is autoimmune (Hashimoto's).
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Hypothyroidism develops when the thyroid gland produces too little thyroid hormone (T4 and T3). The pituitary detects this and raises TSH to try to drive output back up — which is why TSH is the central diagnostic and monitoring marker. In iodine-replete countries the dominant cause is Hashimoto's thyroiditis, an autoimmune attack on thyroid tissue. Worldwide, iodine deficiency still drives a substantial share. Other causes include post-treatment hypothyroidism (after radioactive iodine or thyroidectomy), drug-induced (amiodarone, lithium), and central / pituitary failure (rare).
Key lab markers
- TSH — elevated in primary hypothyroidism. The most sensitive single test.
- Free T4 — confirms overt vs subclinical disease.
- Free T3 — adds confirmation in select cases.
- TPO and TG antibodies — distinguish autoimmune (Hashimoto's) from non-autoimmune causes.
Symptoms
- Fatigue
- Cold intolerance
- Weight gain
- Constipation
- Dry skin and hair
- Slowed mentation
- Bradycardia
- Menstrual irregularity
- Goitre (enlarged thyroid)
Related symptoms
Related articles
- Subclinical Hypothyroidism: Understanding Blood Test Results
- Decoding Blood Tests for Hypothyroidism
- Understanding Blood Tests for Hypothyroidism
- Hypothyroidism vs Hyperthyroidism: How to Tell Them Apart
- Hypothyroidism: Causes, Risk Factors, and Management
- Which Blood Tests Detect Hypothyroidism? Key Markers Explained
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When to discuss with a doctor
TSH >4.0 mIU/L on confirmation deserves a primary-care work-up. Levothyroxine replacement is straightforward; the catch is monitoring (6–8 weeks after every dose change, then yearly when stable). Pregnancy needs tighter targets (TSH <2.5 in first trimester). Symptomatic patients with TSH at the upper end of normal also benefit from a thyroid panel + antibody screen.