15.2 Thyroid & Parathyroid Disease

15.2 Thyroid & Parathyroid Disease

Quick Answer: Iodide trap → TPO organification/coupling → T4/T3; T4 converts to active T3 peripherally. Graves = stimulating TSH-receptor Ab; Hashimoto = destructive autoimmunity. PTH ↑Ca2+/↓PO4 via bone, kidney, and 1,25-D; PTHrP drives many malignant hypercalcemias. Know thyroid cancer associations and MEN1/2 patterns.

Thyroid and parathyroid content on the CBSE rewards stepwise physiology: where iodine is handled, which antibody stimulates versus destroys, how calcium is defended minute-to-minute, and which neoplasm or MEN syndrome matches the vignette’s "extra" findings.

Thyroid Hormone Synthesis and Activation

Follicular cells synthesize thyroid hormone on thyroglobulin in the colloid.

1. Iodide trapping: Basolateral Na+/I− symporter (NIS) concentrates iodide (stimulated by TSH). Competitive anions (perchlorate historically) can block uptake.
2. Oxidation and organification: Thyroid peroxidase (TPO) oxidizes iodide and iodinates tyrosyl residues on thyroglobulin → MIT and DIT.
3. Coupling: TPO couples MIT+DIT → T3 and DIT+DIT → T4 (thyroxine is the major secreted product).
4. Endocytosis and proteolysis: Stimulated follicular cells take up colloid, free T4/T3, and release them into blood bound mainly to TBG (also transthyretin and albumin).
5. Peripheral activation: Type 1 and type 2 deiodinases convert T4 → active T3; type 3 deiodinase produces inactive reverse T3 (rT3). Illness can raise rT3 (euthyroid sick patterns).

TSH from the anterior pituitary acts via Gs-cAMP on follicular cells to promote every step of synthesis and growth. TRH from the hypothalamus stimulates TSH. Free T3/T4 provide negative feedback on TRH and TSH.

T3 binds nuclear thyroid hormone receptors and regulates basal metabolic rate, cardiac β-receptor expression, bone turnover, and CNS development (congenital hypothyroidism → cretinism if untreated).

Drug pearl: Thionamides inhibit TPO. PTU also inhibits peripheral T4→T3 (useful in thyroid storm conceptually) and is preferred in first-trimester pregnancy outlines; methimazole is often preferred otherwise. High-dose iodide can transiently inhibit organification (Wolff–Chaikoff); escape occurs in normal glands. Jod–Basedow is iodine-induced hyperthyroidism in autonomous nodules.

Hyperthyroidism and Hypothyroidism Mechanisms

Graves Disease

Autoimmune thyroid-stimulating immunoglobulins (TSIs) activate the TSH receptor → diffuse toxic goiter, high T3/T4, suppressed TSH. Unique findings: orbitopathy (TSH-receptor antigens in orbital fibroblasts), pretibial myxedema, thyroid bruit. Radioiodine uptake is diffusely increased.

Toxic Adenoma / Toxic Multinodular Goiter

Autonomous follicular nodules secrete thyroid hormone independent of TSH (somatic activating mutations in TSH-receptor/Gs pathways often invoked). Uptake is focal ("hot" nodule) or patchy; surrounding tissue is suppressed.

Hashimoto Thyroiditis

Most common cause of hypothyroidism in iodine-sufficient regions. Anti-TPO and anti-thyroglobulin antibodies; lymphocytic infiltration, Hurthle cells, germinal centers. Early destructive phase can release hormone (hashitoxicosis), then permanent hypothyroidism with high TSH and low free T4. Association with other autoimmune disease and slight increased thyroid lymphoma risk in longstanding goiters is classic trivia.

Other hypothyroidism causes

Iodine deficiency (worldwide), iatrogenic (surgery, radioiodine, drugs such as lithium, amiodarone), central hypothyroidism (low/normal TSH with low free T4).

Thyroid Storm and Myxedema Concepts

Thyroid storm: Extreme hyperthyroid physiology—fever, tachycardia/arrhythmia, altered mental status, often precipitated by infection, surgery, or iodine load. Pathophysiology is severe catecholamine sensitivity and metabolic acceleration; treatment concepts block synthesis (thionamides), block release (iodide after synthesis block), block T4→T3 (PTU, β-blockers, steroids), and supportive care.

Myxedema coma: Severe hypothyroidism—hypothermia, hypoventilation, hyponatremia, bradycardia, altered mentation. Non-pitting edema from glycosaminoglycan deposition. Treat with thyroid hormone and supportive care; rule out concurrent adrenal insufficiency before aggressive T3 in some protocols (exam-level awareness).

Thyroid Cancers (Associations Matter)

Exam habit: Calcitonin or amyloid → medullary. Psammoma + orphan annie → papillary. Hematogenous spread emphasis → follicular. Older patient, rock-hard rapid mass → anaplastic. Always link medullary to MEN2 and pheochromocytoma screening conceptually.

Calcium Regulation: PTH, PTHrP, Vitamin D, Calcitonin

Parathyroid hormone (PTH) from chief cells responds within minutes to low ionized Ca2+ (calcium-sensing receptor, CaSR). PTH:

• Increases bone resorption (osteoclast activation via osteoblast RANKL axis)
• Increases renal Ca2+ reabsorption (distal nephron)
• Decreases renal phosphate reabsorption (proximal tubule → phosphaturia)
• Stimulates renal 1α-hydroxylase → more 1,25-dihydroxyvitamin D, which increases intestinal Ca2+ and PO4 absorption

Net PTH effect: ↑ serum Ca2+, ↓ serum PO4 (and ↑ urinary cAMP/phosphate).

PTHrP activates the same PTH/PTHrP receptor—classic in humoral hypercalcemia of malignancy (squamous cell cancers, etc.): high Ca2+, low PTH, low/normal 1,25-D often, phosphaturia.

Vitamin D deficiency / CKD: impaired 1,25-D → low Ca2+ absorption → secondary hyperparathyroidism; CKD also retains phosphate, further driving PTH.

Calcitonin from C cells opposes high Ca2+ by inhibiting osteoclasts; relatively minor chronic regulator in adult humans but a tumor marker in medullary carcinoma.

Primary hyperparathyroidism: usually parathyroid adenoma; hyperplasia in MEN1/MEN2A. High or nonsuppressed PTH with hypercalcemia is the biochemical key.

Secondary: physiologic PTH rise—CKD most tested; also vitamin D deficiency. Calcium is low or normal, not high (if calcium is high, think tertiary or primary).

Hypoparathyroidism: post-thyroidectomy accidental parathyroid removal/injury; autoimmune; DiGeorge (no parathyroids). Low PTH, low Ca2+, high PO4; neuromuscular irritability.

Pseudohypoparathyroidism (type 1a, Albright hereditary osteodystrophy): end-organ Gsα resistance to PTH → high PTH, low Ca2+, high PO4, plus short stature, short 4th/5th metacarpals when maternally inherited allele patterns apply (GNAS imprinting)—high-yield molecular pearl.

MEN Syndromes

Prophylactic thyroidectomy concepts in MEN2 reflect nearly inevitable MTC risk—know the association more than surgical timing details.

Putting Thyroid and Parathyroid Together on Exams

A diffusely hot thyroid with orbitopathy is Graves, not a hot nodule. A cold nodule raises cancer concern (most cold nodules still benign, but cancers are cold). After neck surgery, acute tetany points to hypoparathyroidism, not hypothyroidism alone. Hypercalcemia with suppressed PTH is not primary hyperparathyroidism—look for PTHrP or osteolytic mets or vitamin D excess. Hypercalcemia with high PTH is primary (or tertiary). Medullary cancer plus pheo screams MEN2; parathyroid plus pituitary plus gastrinoma screams MEN1.

Wire synthesis steps to drug targets, antibodies to disease direction (stimulate vs destroy), calcium labs to the driving hormone, and tumors to their molecular/syndromic tags—and this section becomes a reliable point bank on the reproductive–endocrine slice of the CBSE.