Thyroid Functional Disorders

Overview

Embryology: midline tongue-root invagination→descends anterior to trachea. Failed descent=LINGUAL THYROID. Thyroglossal duct disappears by 6wk embryo — proximal remnant=foramen caecum, distal=pyramidal lobe; persistence→THYROGLOSSAL CYST. C-cells=neuroectoderm origin. Adult gland 15-40g, 2 lobes+isthmus.

Histology: colloid-filled follicles(cuboidal epithelium, thyroglobulin colloid)=functional unit. C-cells dispersed(silver stain/IHC only).

Function: T3/T4=metabolic rate. HIGHLY LABILE gland — 3 epithelial phases: RESTING(large follicles, flattened cells, dense colloid — colloid goitre/iodine-treated hyperthyroidism), SECRETORY(cuboidal, normal thyroid), RESORPTIVE(columnar, vacuolated/scalloped colloid — hyperthyroidism).

Hormone synthesis (TSH-regulated): iodine trapping(>20x concentration)→oxidation(thyroid peroxidase)→iodination(tyrosine residues→MIT/DIT)→coupling(→T3/T4)→release(colloid endocytosis+thyroglobulin proteolysis)→circulate bound to thyroxine-binding globulin.

Tests: serum T3/T4(RIA), TSH/TRH, calcitonin, thyroglobulin, RAIU, cold/hot nodule assessment. FNAC=widely used, avoids unnecessary biopsy.

Hyperthyroidism (thyrotoxicosis)

Hypermetabolic state, ↑thyroid hormone. More FEMALES. Both T3+T4 ↑, T3 proportionally MORE.

Etiopathogenesis — 3 COMMONEST: GRAVES’ DISEASE, toxic multinodular goitre, toxic adenoma. Less common: pituitary TSH tumour, TRH hypersecretion, thyroiditis, metastatic thyroid tumours, struma ovarii, congenital(neonate of Graves’ mother), hCG-secreting tumours(hydatidiform mole/choriocarcinoma/testicular — mild thyrotropic effect), excess hormone/iodine intake(JOD-BASEDOW DISEASE).

Clinical: slow insidious onset. Emotional instability, nervousness, palpitations, fatigue, weight loss DESPITE good appetite, heat intolerance, perspiration, menstrual disturbance, fine tremor. Cardiac: tachycardia+cardiomegaly(INVARIABLE). Skin: warm/moist/flushed. ↓skeletal muscle strength+osteoporosis. EXOPHTHALMOS=typical of Graves’ specifically. Labs: ↑T3/T4, suppressed TSH.

THYROID STORM/CRISIS — sudden severity spike: post-subtotal thyroidectomy WITHOUT adequate pre-control, OR acute stress/trauma/infection in existing hyperthyroid patient (PREVENTABLE — argues for pre-op optimisation). High fever, tachycardia, arrhythmias, coma→death from CHF/hyperpyrexia.

Hypothyroidism

Hypometabolic state, inadequate hormone (rarely resistance). 2 forms by AGE OF ONSET (timing = dramatically different consequences):

Cretinism (congenital)

Severe hypothyroidism at birth/first 2yrs — CRITICAL brain-development window, untreated=IRREVERSIBLE physical+mental retardation (contrast reversible myxoedema).

Etiopathogenesis: developmental anomalies(agenesis, ectopic), genetic synthesis defects(trapping/oxidation/iodination/coupling/thyroglobulin), fetal iodide/antithyroid drug exposure, ENDEMIC cretinism(iodine-deficient regions) vs SPORADIC(developmental/genetic).

Clinical: evident weeks-months post-birth — slow to thrive, poor feeding, constipation, dry scaly skin, hoarse cry, bradycardia. Full picture: impaired growth/dwarfism, round face, narrow forehead, widely-set eyes, flat broad nose, protuberant tongue+abdomen. Neuro features(deaf-mutism, spasticity, mental deficiency)=MORE marked in sporadic type. Labs: ↑TSH, ↓T3/T4.

Myxoedema (adult)

Non-pitting oedema — mucopolysaccharide accumulation in dermis/tissue ground substance.

Etiopathogenesis — 2 COMMONEST: thyroid ablation(surgery/radiation), autoimmune/lymphocytic thyroiditis(PRIMARY IDIOPATHIC MYXOEDEMA). Others: endemic/sporadic goitre, hypothalamic-pituitary lesions, thyroid cancer, prolonged antithyroid drugs, mild developmental/dyshormonogenesis.

Clinical: slow onset, full syndrome after YEARS. Cold intolerance, mental/physical lethargy, constipation, slowed speech/intellect, facial puffiness, hair loss, altered skin texture. Labs: ↓T3/T4, MARKEDLY ↑TSH (EXCEPT hypothalamic-pituitary disease=LOW TSH — key localising distinction).

Clinical relevance

3 epithelial phases = testable histologic distinction (resting-phase in iodine-treated vs resorptive-phase in untreated hyperthyroidism). Cretinism vs myxoedema = timing distinction with DRAMATICALLY different consequences — same deficiency causes IRREVERSIBLE neurodevelopmental damage before brain window closes (cretinism) vs REVERSIBLE metabolic syndrome after (myxoedema) — makes neonatal screening the HIGHEST-VALUE intervention here. TSH level = KEY localising discriminator (↑TSH+↓T3/T4=primary thyroid failure; ↓TSH+↓T3/T4=hypothalamic-pituitary problem) — one number pair does most diagnostic work. Thyroid storm’s specific trigger profile = preventable, argues for pre-op optimisation.

Overview

The thyroid arises embryologically from a midline tongue-root invagination, descending in front of the trachea/thyroid cartilage; failed descent causes lingual thyroid. The connecting thyroglossal duct normally disappears by the 6th embryonic week — its proximal remnant is the foramen caecum, distal remnant the pyramidal lobe; persistent remnants can form a thyroglossal cyst. C-cells derive from neuroectoderm. The adult gland weighs 15–40 g, two lateral lobes joined by an isthmus (± pyramidal lobe).

Histologically, colloid-filled follicles (lined by cuboidal epithelium, microvilli extending into thyroglobulin-containing colloid) are the functional units, separated by delicate vascularised fibrous tissue; calcitonin-secreting C-cells are dispersed within follicles (identifiable only by silver stains/IHC).

Function: thyroxine (T4) and tri-iodothyronine (T3) maintain metabolic rate. The thyroid is highly labile, responding to puberty, pregnancy, stress, and pathologic states — reflected in three epithelial phases: resting (large follicles, flattened cells, dense homogeneous colloid — e.g. colloid goitre, iodine-treated hyperthyroidism), secretory (cuboidal epithelium, moderately dark colloid — normal thyroid), resorptive (columnar epithelium, lightly-stained vacuolated/scalloped colloid — hyperthyroidism).

Hormone synthesis (TSH-regulated): iodine trapping (>20-fold concentration) → oxidation by thyroid peroxidase → iodination of thyroglobulin tyrosine residues (forming MIT, DIT) → coupling (forming T3, T4) → release via colloid endocytosis and thyroglobulin proteolysis, with T3/T4 then circulating bound to thyroxine-binding globulin.

Thyroid function tests: serum T3/T4 (RIA), TSH/TRH levels, calcitonin, thyroglobulin, radioactive iodine uptake (RAIU), and cold-vs-hot nodule functional assessment. FNAC is now widely used to avoid unnecessary biopsies.

Hyperthyroidism (Thyrotoxicosis)

A hypermetabolic clinical/biochemical state from excess thyroid hormone production; more frequent in females; both T3 and T4 rise, with T3 rising proportionally more.

Etiopathogenesis — three most common causes: Graves’ disease, toxic multinodular goitre, and toxic adenoma. Less frequent: pituitary TSH-secreting tumour, TRH hypersecretion, thyroiditis, metastatic thyroid tumours, struma ovarii, congenital hyperthyroidism (neonate of a Graves’ mother), hCG-secreting tumours (hydatidiform mole, choriocarcinoma, testicular tumours — via hCG’s mild thyrotropic effect), and excessive thyroid hormone/iodine intake (Jod-Basedow disease).

Clinical features: slow, insidious onset, variable severity — emotional instability, nervousness, palpitations, fatigue, weight loss despite good appetite, heat intolerance, perspiration, menstrual disturbance, fine tremor. Cardiac: tachycardia, palpitations, cardiomegaly (invariable). Skin: warm, moist, flushed. Skeletal muscle weakness and osteoporosis are common. Exophthalmos is typical of Graves’ disease specifically. Labs: elevated T3/T4, suppressed TSH.

Thyroid storm (thyroid crisis) — a sudden severity spike in patients undergoing subtotal thyroidectomy before adequate control, or under acute stress/trauma/severe infection: high fever, tachycardia, arrhythmias, coma — may die of CHF or hyperpyrexia.

Hypothyroidism

A hypometabolic state from inadequate thyroid hormone production (prolonged), or rarely hormone resistance. Two forms by age of onset:

Cretinism (Congenital Hypothyroidism)

Severe hypothyroidism at birth or within the first 2 postnatal years — the critical brain-development window; untreated, causes physical and mental retardation. (The name derives from French, “Christ-like,” reflecting the severity of retardation.)

Etiopathogenesis: developmental anomalies (thyroid agenesis, ectopic thyroid); genetic hormone-synthesis defects (iodine trapping/oxidation/iodination/coupling/thyroglobulin synthesis defects); fetal iodide/antithyroid drug exposure; endemic cretinism (regions with endemic goitre, dietary iodine lack) — versus sporadic cretinism (developmental/genetic causes).

Clinical features: evident within weeks to months of birth — slow to thrive, poor feeding, constipation, dry scaly skin, hoarse cry, bradycardia. Fully-developed picture: impaired skeletal growth/dwarfism, round face, narrow forehead, widely-set eyes, flat broad nose, big protuberant tongue, protuberant abdomen. Neurological features (deaf-mutism, spasticity, mental deficiency) are more marked in sporadic cretinism. Labs: raised TSH, low T3/T4.

Myxoedema (Adult Hypothyroidism)

Non-pitting oedema from hydrophilic mucopolysaccharide accumulation in dermal/other tissue ground substance.

Etiopathogenesis — commonest two causes: thyroid ablation (surgery/radiation) and autoimmune (lymphocytic) thyroiditis (primary idiopathic myxoedema). Other causes: endemic/sporadic goitre, hypothalamic-pituitary lesions, thyroid cancer, prolonged antithyroid drug use, mild developmental anomalies/dyshormonogenesis.

Clinical features: slow onset, full syndrome after years — cold intolerance, mental/physical lethargy, constipation, slowed speech/intellect, facial puffiness, hair loss, altered skin texture. Labs: low T3/T4, markedly raised TSH (except hypothalamic-pituitary disease, where TSH is low).

Clinical Relevance

• The three thyroid epithelial phases (resting/secretory/resorptive) are worth anchoring to specific clinical scenarios — resting-phase morphology in iodine-treated hyperthyroidism versus resorptive-phase morphology in untreated hyperthyroidism is a subtle but genuinely testable histologic distinction.
• Cretinism-vs-myxoedema is fundamentally a timing distinction with dramatically different consequences — the same underlying hormone deficiency causes irreversible neurodevelopmental damage before the brain-development window closes (cretinism) versus a reversible metabolic syndrome afterward (myxoedema), making early neonatal screening the single highest-value intervention in this entire topic.
• TSH level is the key discriminator for localising hypothyroidism’s cause: elevated TSH with low T3/T4 points to primary thyroid failure, while low TSH with low T3/T4 points to a hypothalamic-pituitary problem — this single pairing of numbers does most of the diagnostic work.
• Thyroid storm’s specific trigger profile (recent subtotal thyroidectomy without adequate pre-control, or acute stress/infection in an existing hyperthyroid patient) is worth remembering as a preventable complication — the emphasis on “before adequate control” is a direct argument for pre-operative optimisation in thyroid surgery.

• Thyroid hormone synthesis pathway (iodine trapping → oxidation → iodination → coupling → release) as a step-by-step flow diagram
• Cretinism vs myxoedema timeline showing the critical brain-development window and why the same deficiency produces irreversible vs reversible outcomes
• TSH/T3/T4 decision table showing how the three lab values localise hyper/hypothyroidism to its cause