Thyroid → Metabolism → Heart Rate → Weight → Energy
The biology
The pituitary in the brain releases TSH to tell the thyroid how much T4 and T3 to make. T3 sets the speed of every cell — heart rate, body temperature, gut movement, mood.
What this means
High TSH with low T4 means an under-active thyroid — slow, cold, tired, gaining weight. Low TSH with high T4/T3 means over-active — fast heart, weight loss, anxiety.
Advice
Hypothyroidism — daily levothyroxine usually fixes it. Hyperthyroidism — needs urgent specialist care to protect the heart.
Tests in this chain