Thyroid-Stimulating Hormone (TSH)

Two groups read these cutoffs differently. TSH drifts upward in older adults as a normal finding, so a mildly elevated value at 78 is read differently than the same number at 28. And pregnancy rewrites the rules entirely: TSH runs lower in the first trimester, and obstetric care uses its own trimester-specific ranges.

A confirmed high TSH means the pituitary is compensating for a thyroid that's falling behind. The common reasons:

• Hashimoto's thyroiditis, an autoimmune condition where the immune system gradually wears the thyroid down. It's by far the most common cause where dietary iodine is adequate, and a positive anti-TPO antibody test confirms the immune involvement behind it.
• Recovering thyroiditis: temporary inflammation after a viral illness or postpartum can swing TSH high during recovery, then settle on its own.
• Medications, with lithium and amiodarone the classic examples.
• Under-replacement, if you already take levothyroxine and the current dose is no longer enough.
• Iodine deficiency, rare where salt is iodized and still relevant elsewhere.

Symptoms of an underactive thyroid develop slowly and read like a dimmer switch turned down: fatigue, feeling cold, weight gain without diet change, constipation, dry skin, thinning hair, brain fog, low mood, heavier periods.

The borderline zone, a TSH of 4–10 with normal Free T4, is called subclinical hypothyroidism and is genuinely gray. Guidelines favor repeating the test in 6–12 weeks with antibodies before deciding anything. Persistent elevation plus symptoms, antibodies, or pregnancy plans tilt the decision toward treating; an isolated wobble usually means watchful waiting.

A suppressed TSH means the pituitary senses too much thyroid hormone. The usual suspects:

• Graves' disease, an autoimmune condition that switches the thyroid into overdrive and the most common cause of hyperthyroidism
• overactive thyroid nodules, patches of tissue producing hormone on their own schedule
• thyroiditis in its early phase, when an inflamed thyroid leaks stored hormone
• a levothyroxine dose that's now too high, which does this quietly and commonly
• sometimes no thyroid problem at all, just biotin interference (see the FAQ)

An overactive thyroid feels like the body's idle speed set too high: heart palpitations, heat intolerance and sweating, unintended weight loss, tremor, anxiety, poor sleep, lighter periods. A persistently suppressed TSH deserves attention even without dramatic symptoms, because over time it strains heart rhythm and bone density.

TSH is the smoke detector; the rest of the thyroid panel tells you whether and where there's fire. Free T4, and sometimes Free T3, shows actual hormone output, which separates "subclinical" from overt disease and helps spot the rare cases where the pituitary itself is the problem. The TSH vs Free T4 comparison walks through the four combinations, and the thyroid panel guide assembles the full reading order.