Hypothyroidism Diagnosis: Which Tests Confirm It

The diagnostic sequence

1. TSH — the sensitive screen; values >4.0 (lab-dependent) prompt investigation
2. Free T4 — distinguishes overt (low T4, treat now) from subclinical (normal T4, discuss monitoring)
3. TPO antibodies — positive results confirm autoimmune Hashimoto's mechanism, predicting progression probability in subclinical cases
4. Repeat confirmation — 6–8 weeks later excluding transient spikes before lifelong commitments

Symptoms that justify testing

Fatigue disproportionate to sleep, cold intolerance, unexplained weight gain, constipation, dry skin, hair thinning (especially outer eyebrow loss), heavy periods, depression/anxiety, high cholesterol surprise findings, bradycardia. Any three together justify a panel regardless of age; women over 35 earn screening by demographics alone (1-in-8 lifetime risk).

Subclinical hypothyroidism — treat or watch?

The genuinely contested zone: TSH 4–10 with normal hormones. Factors favoring treatment: TPO-antibody positivity, symptoms clearly attributable, pregnancy/planning (treat liberally), TSH >10. Factors favoring watching: elderly patients, antibody-negative, mild deviations. Repeat-in-6-weeks resolves many debates as transient recoveries self-normalize.

Treatment monitoring cadence

Levothyroxine starters recheck TSH at 6–8 weeks (full steady-state timing), adjust doses in 12.5–25mcg steps, stabilize, then annually. Absorption variables (calcium/iron/PPI co-ingestion, morning-vs-evening consistency) explain stubborn 'unresponsive' cases more often than dosage math errors.