STEPTRACKERENDOCRINE | PATHOLOGY + IMMUNOLOGY + EMBRYOLOGY | HIGH-YIELD REVIEW

Hypothyroidism & Thyroiditis

An underactive thyroid from autoimmunity, iodine lack, development or drugs — and the inflammatory thyroiditides that swing from high to low.

FOCUS ON
Hashimoto: anti-TPO, lymphocytes with germinal centers and Hürthle cells, painless goiter → hypothyroidism; ↑ thyroid lymphoma risk. Subacute (de Quervain): after a virus, painful gland, ↑ ESR, granulomas, low uptake. Painless/postpartum: transient hyper → hypo. Riedel: rock-hard fibrosis (IgG4). Congenital: dysgenesis → newborn screening.
DISORDER · TOPIC
MECHANISM
FEATURES
CLUE
Hypothyroidism — features
↓ BMR, ↓ β-adrenergic tone, GAG accumulation (myxedema)
Cold intolerance, weight gain, fatigue, constipation, bradycardia, dry skin, hair loss, hoarseness, facial/periorbital edema, slow-relaxing reflexes, menorrhagia
↑ LDL, ↑ CK, hyponatremia, ↑ prolactin
Hashimoto thyroiditis
most common (iodine-sufficient)
Autoimmune (type IV + antibodies): anti-TPO, anti-thyroglobulin; HLA-DR3/DR5
Middle-aged women; painless goiter, may start with transient "hashitoxicosis"; lymphoid follicles with germinal centers, Hürthle cells
↑ thyroid B-cell lymphoma (marginal zone)
Iodine deficiency
most common worldwide
Not enough substrate → ↑ TSH → goiter
Endemic goiter, hypothyroidism
Iodized salt
Congenital hypothyroidism
Thyroid dysgenesis (most), dyshormonogenesis, maternal iodine deficiency or antibodies
Pot belly, pallor, puffy face, umbilical hernia, large tongue, prolonged jaundice, poor feeding, intellectual disability
Newborn screen; treat early → normal IQ
Subacute granulomatous (de Quervain)
Post-viral inflammation
Painful, tender thyroid, fever, ↑ ESR; granulomas with multinucleated giant cells; hyper → hypo → normal
Low uptake · NSAIDs, β-blockers
Painless (silent) thyroiditis
Autoimmune lymphocytic inflammation; anti-TPO
Painless small goiter; transient thyrotoxicosis then hypothyroidism
Low uptake, normal ESR
Postpartum thyroiditis
Immune rebound after pregnancy
Within a year of delivery; hyper → hypo phase; recurs
Anti-TPO ⁺ · may become permanent
Riedel thyroiditis
IgG4-related; fibrosis extends beyond the capsule
Rock-hard, fixed, painless goiter; compresses trachea, nerves; hypothyroidism in some
Mimics anaplastic carcinoma · steroids
Drug-induced
Iodine load (Wolff-Chaikoff), interference with synthesis or immunity
Amiodarone, lithium, iodinated contrast, interferon-α, immune checkpoint inhibitors
Amiodarone can also cause hyperthyroidism
Myxedema coma
Severe untreated hypothyroidism + stressor (infection, cold, sedatives)
Hypothermia, bradycardia, hypoventilation, hyponatremia, coma
IV levothyroxine + hydrocortisone
Thyroglossal duct cyst
Persistent duct from foramen cecum
Midline neck mass that moves up with swallowing or tongue protrusion
vs branchial cleft cyst: lateral, by SCM

Mechanism — thyroid hormone synthesis

Iodide uptakeNa⁺/I⁻ symporter
→
TPOOxidation, organification, coupling
→
Stored on thyroglobulin
→
T4 (mostly) + T3 released
→
5′-deiodinaseT4 → T3 in tissues

Hashimoto antibodies target TPO and thyroglobulin; thionamides block TPO; PTU, steroids and propranolol block peripheral conversion.

Reading thyroid labs

STATE
TSH
FREE T4
NOTE
Primary hypothyroid
↑
↓
Hashimoto, iodine
Subclinical
↑
Normal
Often anti-TPO ⁺
Central hypothyroid
↓ / normal
↓
Pituitary, hypothalamus
Sick euthyroid
Normal / ↓
Normal / ↓
↓ T3, ↑ reverse T3
Thyroiditis (early)
↓
↑
Low uptake

How it's tested

COMMON CONFUSIONSubacute thyroiditis hurts and has a high ESR; painless and postpartum thyroiditis don't. All three have low radioiodine uptake in the thyrotoxic phase.
THE TRAPA rapidly enlarging thyroid mass in a woman with long-standing Hashimoto is lymphoma until proven otherwise.
YOUR APPROACHTSH + free T4 → primary vs central → antibodies (TPO) → pain, ESR, timing (post-viral, postpartum) → gland texture (rubbery, tender, rock-hard).

If you see… think…

Woman with fatigue, weight gain, ↑ TSH, anti-TPO⁺ → Hashimoto thyroiditis
Lymphoid germinal centers and Hürthle cells in thyroid → Hashimoto
Hashimoto patient with a rapidly enlarging neck mass → thyroid lymphoma
Tender thyroid and fever after a flu, ↑ ESR, low uptake → subacute granulomatous thyroiditis
Hyperthyroid then hypothyroid four months after delivery → postpartum thyroiditis
Rock-hard fixed thyroid with ↑ IgG4 → Riedel thyroiditis
Newborn with prolonged jaundice, big tongue, umbilical hernia → congenital hypothyroidism
Hypothermic, bradycardic, confused older woman with myxedema → myxedema coma
Hypothyroidism on bipolar therapy → lithium
Midline neck cyst that rises with tongue protrusion → thyroglossal duct cyst