Hypothyroidism
A state of thyroid hormone deficiency that slows metabolic rate in virtually every tissue, so the whole clinical picture (from bradycardia to cold intolerance to slowed cognition) can be predicted simply by asking what happens when metabolism runs too slowly.
Definition
Hypothyroidism is deficiency of thyroid hormone, most often primary (thyroid gland failure) with a raised TSH and low free T4. Subclinical hypothyroidism is a raised TSH with a normal free T4.
First principles
Thyroid hormone sets the metabolic idle speed of almost every cell
Thyroxine (T4) and triiodothyronine (T3) increase basal metabolic rate, oxygen consumption and protein turnover across nearly all tissues. When levels fall, cellular metabolism slows down uniformly rather than in one organ, which is why hypothyroidism produces such a broad, multisystem picture: every symptom is really the same mechanism (reduced metabolic drive) expressed in a different organ.
This page is exam revision material, not medical advice, and must not be used for patient care. Always check drug doses against the BNF and current guidance. Full disclaimer.