TSH Feedback Loop

• Thyroid stimulating hormone (TSH) is a protein hormone released from the pituitary gland in the brain that stimulates the thyroid to produce thyroid hormone, which regulates body metabolism. TSH is released from the pituitary in response to the release of TRH (thyroid releasing hormone) from the hypothalamus (also in the brain). In a negative feedback loop, thyroid hormone sends a regulatory signal to the hypothalamus to decrease TRH levels.
  
  

Hyperthyroidism and Hypothyroidism

• Two opposite conditions can occur with high TSH levels: the body either produces too much thyroid hormone (hyperthyroidism) or not enough (hypothyroidism). Hyperthyroidism is a condition in which the metabolism speeds up. A person can lose weight, have a fast heartbeat, sweat excessively, be intolerant of heat, become anxious and have difficulty concentrating, and have osteoporosis and diarrhea. Hypothyroidism is the opposite condition, a slower metabolism with weight gain, slow heartbeat, cold skin, cold intolerance, stunted growth and constipation.
  
  

Thyroid

• If the thyroid gland is not able to produce thyroid hormone, the body will keep sending it signals to make the hormone, just as a bill collector keeps asking for money. That signal is TSH, and the commonest cause of a high TSH is hypothyroidism. The thyroid may not make thyroid hormone if there is not enough iodine in the diet (cretinism), or if the thyroid comes under immune attack (Hashimoto's disease), is surgically removed, or becomes fibrosed (Riedel's thyroiditis) or inflamed with a viral infection (de Quervain's thyroiditis). There is a small risk of thyroid cancer with higher levels of TSH.
  
  

Pituitary and Hypothalamus

• The pituitary and hypothalamus are part of the brain. A rare pituitary tumor (pituitary adenoma) can create too much TSH. Even less commonly, a tumor forms in the hypothalamus, releasing TRH, in turn causing TSH levels to rise.
  
  

Diagnosing Hypothyroidism

• The problem can be identified if the levels of thyroid hormone and iodine are measured. Low thyroid hormone levels suggest the thyroid is not functioning normally. Iodine levels can also be checked. A thyroid scan measures whether the thyroid is functioning. Fortunately, thyroid hormone can be replaced, correcting the TSH levels.
  
  

Diagnosing Hyperthyroidism

• If there is a tumor in the pituitary (rare), levels of TRH will be low because high levels of thyroid hormone will tell the hypothalamus to stop producing TRH. If there is a tumor in the hypothalamus (very rare), then levels of both TRH and thyroid hormone will be high. Practically however, a combination of a CT scan or MRI and a subtype of thyroid hormone called T3 will identify these rare tumors, because TRH cannot be measured.