The thyroid: too little, or too much
Thyroid hormone sets the body's metabolic pace — its tempo for heat, heart rate, and energy. Too little (hypothyroidism) leaves a person cold, slow, and tired; too much (hyperthyroidism) makes them hot, jittery, and fast. The treatments mirror the two faults exactly: replace what is missing, or restrain what is excessive.
For an underactive gland, levothyroxine is a near-perfect replacement: a synthetic copy of the natural hormone, taken once daily as a tablet. Being a small lipid-soluble molecule that acts on a nuclear receptor, it has a long half-life of about a week, so levels stay smooth and one missed dose barely matters. Because the dose must land in a narrow band, it is fine-tuned by [[therapeutic-drug-monitoring|monitoring]] the pituitary's TSH signal in the blood.
For an overactive gland, an antithyroid drug blocks the enzyme that builds thyroid hormone, slowly bringing levels down over weeks. It does not destroy the gland; it simply turns the factory's output dial down while the underlying cause is sorted out.
Corticosteroids: powerful, and respectful of feedback
The adrenal gland makes two families of steroid that drugs imitate. A glucocorticoid (like cortisol) is the body's master anti-inflammatory and stress hormone; a mineralocorticoid (like aldosterone) controls salt and water. Synthetic corticosteroids are among the most useful drugs in medicine — they calm inflammation in asthma, arthritis, allergy, and autoimmune disease — but their power is matched by their hazards.
Used for more than a week or two, a corticosteroid triggers exactly the feedback shutdown from guide 1: the hypothalamic–pituitary axis reads high steroid in the blood and stops driving the adrenal gland, which then goes quiet. Stop the drug abruptly and the body is left with no cortisol — a dangerous adrenal crisis. The rule is absolute: long-term steroids are always tapered down slowly, never stopped at once.