colloid

**1. Thyroid colloid (most high-yield for Step 1)** - Composition: Mainly thyroglobulin (Tg), a large glycoprotein synthesized by follicular cells and secreted into the follicular lumen. Tyrosine residues on Tg are iodinated to form MIT and DIT, which couple to form T3 and T4 while still attached to Tg. - Thyroid hormone synthesis steps: 1. Na+/I- symporter (NIS) traps iodide at the basolateral membrane (secondary active transport). Radioiodine uptake uses this. 2. Pendrin transports iodide into the lumen (apical). Pendrin mutation causes Pendred syndrome (goiter + sensorineural hearing loss). 3. Thyroid peroxidase (TPO) oxidizes iodide (I- to I2), iodinates (organifies) tyrosine on Tg, and couples MIT/DIT (DIT + DIT = T4; MIT + DIT = T3). 4. TSH stimulates endocytosis of colloid. Lysosomes cleave Tg (proteolysis), releasing T3/T4 into blood. MIT/DIT are deiodinated and the iodine is recycled. - Inhibitors: Propylthiouracil (PTU) and methimazole inhibit TPO (organification and coupling). PTU also inhibits peripheral 5'-deiodinase (T4 to T3). The Wolff-Chaikoff effect is an autoregulatory decrease in organification with high iodide load. Perchlorate and thiocyanate competitively inhibit NIS. - Clinical correlations: - Colloid goiter (nodular goiter): Enlarged follicles distended with colloid and flattened epithelium. Seen with iodine deficiency (endemic) or sporadic. Usually euthyroid, and may become toxic (toxic multinodular goiter, Plummer disease). - Graves disease: Scalloped colloid at the periphery with tall columnar, hyperplastic epithelium. - Hashimoto thyroiditis: Lymphocytic infiltrate with germinal centers, Hürthle (oncocytic) cells, and loss of colloid. Anti-TPO and anti-Tg antibodies. - Papillary carcinoma: Orphan Annie eye nuclei, psammoma bodies, and nuclear grooves/pseudoinclusions. Colloid is typically scant or absent. - Follicular adenoma/carcinoma: Follicles with colloid. Carcinoma is distinguished from adenoma by capsular or vascular invasion, not cytology. - Thyroglobulin as a tumor marker: Used for recurrence monitoring after thyroidectomy for differentiated thyroid cancer (papillary and follicular). It is meaningful only after total thyroidectomy and ablation. Anti-Tg antibodies can interfere. - Thyroglossal duct cyst and colloid cysts of the thyroid are other related lesions.

**2. Colloid cyst of the third ventricle (neuro)** - Benign, gelatinous cyst near the foramen of Monro. It can cause obstructive (noncommunicating) hydrocephalus with positional headaches, and rarely sudden death or drop attacks. Treatment is endoscopic resection or shunting.

**3. Colloid (mucinous) carcinoma** - Tumors, for example breast or colorectal, with abundant extracellular mucin pools. In the breast, mucinous carcinoma generally has a better prognosis.

**4. Colloid fluids (physiology and critical care)** - Contain large molecules that don't easily cross capillary membranes, so they raise plasma oncotic pressure and expand intravascular volume. Examples are albumin, dextrans, hydroxyethyl starch, and gelatins. - Crystalloids (normal saline, lactated Ringer's) distribute across extracellular fluid, with about 25% staying intravascular. - Starling forces: Net filtration = Kf[(Pc − Pi) − σ(πc − πi)]. Decreased plasma oncotic pressure (πc), as in nephrotic syndrome, cirrhosis, or malnutrition, causes edema. - Albumin is used in spontaneous bacterial peritonitis (with antibiotics), large-volume paracentesis, and hepatorenal syndrome. Hydroxyethyl starch is associated with AKI and coagulopathy, so it is largely avoided in sepsis.

**5. General chemistry meaning** - A colloid is a suspension of particles (1 nm to 1 μm) that does not settle. Colloid osmotic pressure is the same as oncotic pressure.

**High-yield pearls:** Colloid = Tg storage in follicles. TSH triggers colloid endocytosis. Organification occurs at the apical membrane/lumen interface via TPO. Hormone is still bound to Tg until proteolysis.