Thyroid Level (TSH) Assessment

TSH stands for Thyroid-Stimulating Hormone. It is produced by the pituitary gland in the brain and controls the thyroid gland’s production of T3 (triiodothyronine) and T4 (thyroxine), the hormones that regulate metabolism, energy, body temperature, heart rate, and many other functions.

TSH works in an inverse feedback loop. When the thyroid is underactive and T3/T4 levels are low, the pituitary releases more TSH to stimulate the thyroid to produce more. When the thyroid is overactive and T3/T4 levels are too high, the pituitary reduces TSH output.

This means:

• High TSH = thyroid is underactive (hypothyroidism)
• Low TSH = thyroid is overactive (hyperthyroidism)

Typical reference ranges (mIU/L), which this calculator uses:

Babies under a year run higher, and newborns far higher for the first few days, so read an infant’s result against the lab’s own age range.

Pregnancy is split by trimester because the ranges genuinely move. Human chorionic gonadotropin (hCG) peaks in the first trimester and stimulates the same receptor on the thyroid that TSH does, so the pituitary sees plenty of thyroid hormone and turns its own signal down. A suppressed TSH at 9 weeks is usually normal physiology rather than hyperthyroidism, and it drifts back up as hCG falls. Reading a first-trimester result against the ordinary adult range is one of the more common ways to frighten someone for no reason.

Older pages, this one included until recently, give pregnancy upper limits of 2.5 and 3.0. Those came from the American Thyroid Association’s 2011 guideline. Its 2017 revision dropped them: a lab’s own trimester ranges come first, and without them the upper limit is about 4.0. The 2.5 figure survives in one place, as the level above which treatment may be considered for a woman with positive TPO antibodies.

What about an “optimal” TSH of 1 to 2.5? You will see it a lot. It traces back to 2003 guidance noting that most people without thyroid disease sit under 2.5, and it was never adopted as a treatment target by the endocrine societies. A TSH of 0.6 or 3.5 in an adult is simply normal. That is why this calculator does not grade results inside the range.

The upper limit rising with age is real too. TSH distribution shifts upward across the population after about 70, and a level that would prompt treatment at 40 is often left alone at 80. The TRUST trial (Stott and colleagues, 2017) gave levothyroxine to adults over 65 with TSH between 4.6 and 19.9 and found no improvement in symptoms or tiredness.

Symptoms of high TSH (hypothyroidism) include fatigue, weight gain, cold intolerance, brain fog, constipation, dry skin, and depression.

Symptoms of low TSH (hyperthyroidism) include weight loss, rapid heartbeat, anxiety, sweating, heat intolerance, tremors, and insomnia.

A single TSH test is a starting point, not a complete diagnosis. The usual follow-up is free T4, sometimes free T3, and TPO antibodies. Reverse T3 turns up in commercial “full panels”, but the major guidelines do not use it. Always discuss your results with a licensed healthcare provider, who will weigh your symptoms, history, and full test panel before making any diagnosis or treatment recommendation.

Many factors temporarily affect TSH levels: stress, sleep deprivation, illness, biotin supplements, and the time of day the blood is drawn.