Hypothyroidism
Underactive thyroid — the gland produces too little hormone. The leading global cause in iodine-replete populations is autoimmune (Hashimoto's).
What it is
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)
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.