Hypothyroidism & Myxedema Coma

Thyroid hormone is governed like a thermostat. The hypothalamus releases thyrotropin-releasing hormone (TRH), which tells the anterior pituitary to release thyroid-stimulating hormone (TSH); TSH tells the thyroid gland to make hormone. The gland releases mostly T4 (about 80%) and some T3 (about 20%) — but tissue receptors bind T3 far more avidly, so T4 is relatively inactive until deiodinase enzymes in the liver, kidney, and muscle convert it to active T3 (or to inactive reverse T3). Rising free T4 and T3 then inhibit TRH and TSH — the negative feedback loop that holds levels steady.

Two details from this axis do heavy exam work. First, iodine is an integral part of T3 and T4 — absorbed in the small intestine from iodized salt, seafood, and vegetables — so too little iodine means too little hormone. Second, the pituitary is an exquisitely sensitive amplifier: for a two-fold change in serum T4, TSH can change up to 100-fold in the opposite direction. That amplification is exactly why TSH is the first-line screening test — it moves dramatically while T4 is still drifting.

When the thyroid gland itself fails, T4 falls, the feedback brake is released, and TSH climbs — primary hypothyroidism, the common form. When the pituitary or hypothalamus is the broken part, the gland is normal but never receives the order: TSH is low or inappropriately normal — central (secondary) hypothyroidism, which can never be diagnosed on TSH alone.

Thyroid hormone sets the basal metabolic rate: it increases Na⁺/K⁺-ATPase expression across tissues, raising oxygen consumption, respiration, and body temperature. Take the hormone away and every system decelerates in a predictable way. Less ATPase activity → less oxygen burned → less heat produced — cold intolerance, reduced sweating, and eventually hypothermia. A lower metabolic rate — fatigue and weight gain. In the heart, thyroid hormone normally has a permissive effect on catecholamines, increasing β-receptor expression; without it the heart stops answering adrenaline — bradycardia, reduced contractility, falling cardiac output. In the gut, lost tone and motility mean constipation (and at the extreme, ileus). In the brain, T3 drives wakefulness, alertness, and mood — its loss produces cognitive slowing, somnolence, and depression.

The same slowdown theme covers the findings whose cellular mechanisms the sources don’t spell out: dry coarse skin, brittle hair and hair loss, voice changes, prolonged ankle-reflex relaxation, and menstrual irregularities are all classic features of the hormone-deficient, slowed-down state — recognize them as part of the picture rather than memorizing a separate mechanism for each.

Two named signs deserve their own mechanism. A goiter can form because falling T4 drives TSH up, and relentless TSH stimulation causes hypertrophy and hyperplasia of the thyroid tissue — the gland enlarges trying to obey an order it cannot fill. And myxedema — before it names the coma — names a skin sign: mucopolysaccharides accumulate in skin and soft tissue, producing nonpitting edema and puffiness of the face, hands, and legs, periorbital edema, and an enlarged tongue (macroglossia). That nonpitting quality is the exam contrast with heart failure’s edema: this is tissue infiltration, not hydrostatic fluid.

The lab pattern is the axis translated to numbers. Primary hypothyroidism = elevated TSH with low free T4: the pituitary is screaming at a gland that can’t answer. (Free T4 — not total T4 — is what’s measured, because protein-bound hormone can’t enter tissues.) Central hypothyroidism = low free T4 with a low or inappropriately normal TSH: the order was never sent. Subclinical hypothyroidism = elevated TSH with a normal free T4 — the amplifier has noticed a deficit the T4 assay can’t yet see; it progresses to overt disease in about 2–6% of patients per year, faster (3–8%) when TSH exceeds 10 mIU/L with positive thyroid peroxidase (TPO) antibodies.

Hypothyroidism is common in exactly the pattern its risk factors predict: overt disease in about 0.3% and subclinical in about 4.3% of Americans aged 12 and older, more prevalent in women and with advancing age. The prognosis on treatment is genuinely good — symptoms usually reverse over weeks to months — but untreated disease raises morbidity and mortality, and its far end is the coma described below.

Myxedema coma is the most severe, life-threatening form of hypothyroidism — altered mentation plus multiorgan dysfunction, with mortality around 20–60% (≈ 39% in the most recent pooled analysis). It is rare, and it almost never appears out of nowhere: usually months of somnolence and lethargy in someone untreated or undertreated, then a precipitant tips the balance — infection (most common), cold exposure, sedatives, tranquilizers, opioids, anesthetics, surgery, trauma, stroke, and drugs like amiodarone, lithium, and diuretics. About 90% of cases occur in winter, 80% in women, most over age 60.

Sedatives and opioids deserve their own warning: a hypothyroid patient metabolizes drugs slowly, so standard doses behave like overdoses — the “sleepy older patient” who got routine opioids and is now barely arousable may be sliding into the crisis, not sleeping it off.

The spiral itself is the slowed-metabolism map pushed to failure. Heat production collapses — hypothermia, frequently below 95.9 °F (35.5 °C). The catecholamine-deaf heart slows and weakens — bradycardia and falling contractility, with early compensatory diastolic hypertension giving way to hypotension and shock. The respiratory drive loses its response to hypercapnia; with diaphragmatic weakness and an infiltrated, swollen airway, CO₂ accumulates — CO₂ narcosis is the primary cause of the coma. Meanwhile the kidney moves less water to the distal nephron while ADH runs high — dilutional hyponatremia that directly worsens the mental status — and depressed metabolism drops glucose production, adding hypoglycemia. Every arm feeds the falling level of consciousness.