TSH (thyroid-stimulating hormone)

A thermostat for your thyroid

Think of TSH as a thermostat signal. When thyroid hormone in your blood drops, the pituitary turns up TSH to push the thyroid harder. When thyroid hormone is plentiful, the pituitary turns TSH down. Because of this inverse relationship, TSH moves in the opposite direction from thyroid function.

Symptoms that prompt testing

• Underactive thyroid: fatigue, weight gain, feeling cold, dry skin, thinning hair, constipation, low mood, joint and muscle aches, and heavy or irregular periods.
• Overactive thyroid: weight loss despite eating normally, a racing or irregular heartbeat, anxiety, poor sleep, tremor, heat intolerance and frequent bowel movements.

Why it matters in midlife

NIDDK notes that hypothyroidism is far more common in women and in people over 60, and Hashimoto's disease, an autoimmune condition, is the most common cause. Many symptoms of an underactive thyroid overlap with perimenopause, low testosterone, poor sleep and depression, which is why a TSH test is often part of a midlife work-up for fatigue or weight change.

Even so, the U.S. Preventive Services Task Force found insufficient evidence to recommend screening adults who have no symptoms. Testing is most useful when symptoms or risk factors are present.

Follow-up tests

A TSH result shows whether thyroid function may be off, not why. If it is abnormal, clinicians typically add free T4 and sometimes T3, plus thyroid antibodies to look for Hashimoto's or Graves' disease. Rarely, an abnormal TSH reflects a pituitary problem rather than a thyroid problem.

Things that can shift TSH

• Serious illness unrelated to the thyroid, which can temporarily lower it.
• Older age; TSH tends to run higher in people over 80 without thyroid disease.
• Pregnancy, especially the first trimester.
• Certain medicines, including thyroid hormone, steroids and some heart and psychiatric drugs.
• Biotin supplements, which can interfere with some lab assays.

If you are treated

Levothyroxine is the standard treatment for an underactive thyroid. NIDDK notes that blood tests are usually repeated about 6 to 8 weeks after starting or changing the dose, then less often once stable. A HealifyMD clinician can include TSH in your labs and coordinate care if a thyroid condition is found.

Preparing for a TSH test

TSH can be drawn at any time of day, but testing at a similar time each visit makes trends easier to follow. If you take thyroid medication, ask whether to take your dose before or after the draw.