TSH (thyroid-stimulating hormone)

A pituitary hormone that governs the thyroid. TSH is used as the first value in thyroid diagnostics. How it is to be read is set out in your findings report.

How TSH works

The hypothalamus releases TRH, the pituitary responds with TSH, and TSH stimulates the thyroid to take up iodine, grow and produce the hormones T4 and T3. The thyroid hormones act back on the hypothalamus and pituitary and slow TSH release (negative feedback). Because even small changes in fT4 trigger a marked counter-movement of TSH, TSH often shows changes earlier than the hormones themselves. TSH follows a daily rhythm with higher values at night.

Raised TSH – what happens in the body?

A raised TSH value shows that the pituitary is driving the thyroid harder – usually because there is too little thyroid hormone in the blood. As long as the thyroid can still meet the extra demand, fT4 and fT3 stay within range (subclinical hypothyroidism); when it no longer can, metabolism slows down with feeling cold, lack of drive, constipation and dry skin. A persistently strong TSH stimulus can make the thyroid grow (goitre). Temporarily raised values also occur after severe illness and with iodine deficiency.

Low TSH – what happens in the body?

A low TSH value usually shows that there is enough or too much thyroid hormone in the blood and the pituitary is therefore holding back. If fT4 or fT3 are raised, metabolism speeds up with racing heart, sweating, restlessness and weight loss. Less often TSH is low because the pituitary itself produces too little – then fT4 and fT3 are low as well. Taking thyroid hormone also changes the value, and high-dose biotin can interfere with some measurement methods.