Thyroid in Primary Care: Symptoms, Labs, and How to Read Them (Fast)

Thyroid complaints are common—and vague. Here’s a practical, clinic-friendly way to triage symptoms, choose the right tests, and interpret results quickly.

Common Symptoms Patients Report

• Feeling of fullness in the throat, painless enlargement

• Voice changes

• Energy changes (fatigue, wired/tired)

• Heat/cold intolerance

• Skin or hair changes

• Bowel changes (constipation/diarrhea)

• Mood changes (anxiety/depression)

• Menstrual irregularities, fertility concerns

• Goiter or thyroid nodule history

What Labs to Order

Best first test (most clinics):

• TSH with reflex to free T4 (preferred)

If patients request a “full panel,” consider:

• TSH, free T4, total (or free) T3
  (T3 adds value mainly when TSH is suppressed and you’re confirming hyperthyroidism.)

Autoimmunity (when to check):

• TPO antibodies

  • Order if evaluating Hashimoto’s or subclinical hypothyroidism (TSH↑, FT4 normal)—TPO positivity increases risk of progression to overt hypothyroidism.

  • Not for routine screening in asymptomatic, euthyroid patients.

Pro tip—Biotin:

• Biotin can falsely lower TSH and falsely raise T4/T3.

• Ask patients to hold biotin for 48 hours before labs.

Interpreting Thyroid Labs (simple rules)

Inverse relationship: high TSH → think hypo; low TSH → think hyper.

1) TSH normal

• Reassure; no further testing needed (unless strong clinical red flags).

2) TSH elevated

• Add/confirm free T4.

  • TSH↑ + FT4 low → Overt hypothyroidism → Treat (levothyroxine).

  • TSH↑ + FT4 normal → Subclinical hypothyroidism

    • Discuss and recheck in ~3 months.

    • Consider TPO Ab: positivity predicts progression.

    • Treatment is reasonable if:

      • TSH ≥10 mIU/L, or

      • TSH 4.5–9.9 with symptoms, goiter, positive TPO, pregnancy/pregnancy plans, or cardiovascular risk.

3) TSH low

• Add free T4 and T3.

  • TSH low + FT4/T3 high → Hyperthyroidism (overt)

    • Consider Graves vs thyroiditis vs toxic nodule (signs: tremor, weight loss, tachycardia; Graves may have eye findings).

  • TSH low + FT4/T3 normal → Subclinical hyperthyroidism

    • Repeat in 6–12 weeks; treat if persistent in older adults, atrial fibrillation, osteoporosis, or TSH <0.1.

Don’t miss these:

• Central (secondary) hypothyroidism: Low/normal TSH with low FT4 → think pituitary; evaluate other pituitary hormones and consider imaging.

• Non-thyroidal illness & meds: Severe illness, steroids, amiodarone, lithium, biotin can distort results—interpret in context.

Imaging—When (and what) to order

• Thyroid ultrasound: for palpable nodules, goiter with local symptoms, or abnormal exam—not for lab abnormalities alone.

• RAIU/scan: when TSH is low and you need to distinguish Graves (diffuse uptake) vs toxic nodule(s) (focal uptake) vs thyroiditis (low uptake). Avoid if pregnant.

Quick Start Treatment Notes (thumbnail)

• Overt hypothyroidism: Levothyroxine ~1.6 mcg/kg/day (healthy adult); start lower in older adults/CV disease. Recheck TSH in 6–8 weeks, adjust by 12.5–25 mcg.

• Overt hyperthyroidism: Beta-blocker for symptoms; labs + etiology workup; consider endocrinology for antithyroid meds (methimazole preferred), RAI, or surgery based on cause and patient factors.

Pearls (that save time)

• Re-draw abnormal screens before labeling a diagnosis—confirm away biotin and illness effects.

• TSH changes lag after dose changes; wait 6–8 weeks to recheck after adjusting levothyroxine.

• For symptomatic hyperthyroid patients, start a beta-blocker while sorting the cause.

• Positive TPO + subclinical hypo? Patients appreciate a shared-decision talk about pros/cons of early treatment vs watchful waiting.

Education only; use clinical judgment and local guidelines.

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