Thyroid axis (HPT)
The body's thermostat and idle-speed dial.
TRH and cold in, T₃/T₄ out: basal metabolic rate set
Third verse, same as the first: TRH from the hypothalamus, TSH from the pituitary, T₃ and T₄ from the thyroid, feedback closing the loop. The gland needs dietary iodine to build the hormone at all, and because TSH is also the thyroid's growth signal, anything that keeps TSH (or its impostors) high grows a goiter.
Cold exposure and energy demand raise TRH. TSH is the axis's workhorse signal and the most sensitive thyroid lab value, because it moves opposite to the gland's output.
Follicular cells iodinate thyroglobulin to build the hormone. No dietary iodine, no T₄. The gland exports mostly T₄, a stable courier; deiodinases in target tissues clip it to active T₃. TSH also makes the gland grow, which is the entire story of goiter.
The lesion logic: low T₄ with high TSH is a failing thyroid (primary); low T₄ with low TSH points upstream to the pituitary (secondary). Graves antibodies stimulate the TSH receptor directly, so T₄ runs high while real TSH is suppressed. The feedback still works, but an impostor holds the accelerator.