Parathyroid Hormone (PTH)

Every PTH result is half of a pair. The other half is calcium, because the gland is meant to be reacting to that calcium in real time. Read PTH without it and you are watching a pump run with no idea whether the basement is dry or flooding.

Most US labs report PTH in pg/mL, the same as ng/L, so a "45 pg/mL" and a "45 ng/L" are the same result. The reference interval sits around 15 to 65 pg/mL for the standard intact assay, though it varies by lab and method, so the range printed on your own report is the one to use. What matters is not where PTH falls in that band. It is whether the pump is doing the right thing for the water level beside it.

Here is the trap that catches the most people, and the reason this page exists. A result comes back: PTH 42, sitting comfortably inside the 15 to 65 range, flagged by nothing. Reassuring, until you read one line up and see calcium at 10.8, above its own range.

That combination is not reassuring at all. With calcium running high, working glands should have let the float lift and dropped PTH toward the bottom of the range or below. A PTH that stays mid-range while calcium is elevated is what clinicians call inappropriately normal: normal as a number, wrong as a response. The pump is still running while the basement floods, which means the switch is stuck, not that the water is fine. NIDDK describes exactly this pairing, an elevated or inappropriately normal PTH alongside high calcium, as a common way primary hyperparathyroidism shows up on routine bloodwork.

This is why no honest PTH page can hand you a single number to fear. A 42 can be perfectly fine or quietly abnormal, and only the calcium beside it decides which. The lab does not always flag the pairing, so the calcium and PTH read as a pair is the relationship to understand before reacting to either value alone.

Most high PTH results are not a gland gone rogue. They are glands doing their job well against a steady pull on calcium, which keeps the calcium normal and pushes the effort, the PTH, up. That is secondary hyperparathyroidism, and the high number is the response rather than the problem. When stores fall, rising PTH drives the kidney to convert more of what is left into active vitamin D, which is why the active-D number can look normal in someone clearly deficient.

Keeping PTH elevated for years works as a fix, but it borrows calcium from bone to do it, and over a long stretch that can thin the skeleton. When that overactive remodeling needs following directly, bone-specific alkaline phosphatase reads the pace of bone building rather than its mineral supply, and other turnover markers such as osteocalcin read that same formation pace from a different angle. A persistently high PTH gets traced back to its cause rather than left running.

A low PTH is read the same way, against calcium. If calcium is high and PTH is low, the glands are behaving correctly: they sensed the high calcium and switched off, and the calcium is arriving from somewhere else for the workup to chase. If calcium is low and PTH is also low, that is the worrying pair, because the pump should be running and is not. The usual reason is hypoparathyroidism, classically glands left underactive after neck or thyroid surgery, sometimes alongside a deep magnesium deficit that keeps the glands from releasing hormone at all.

When low calcium does get felt, people describe it as electrical: tingling around the lips and fingertips, muscle cramps, twitching. Those are nerves reacting to a calcium level the glands failed to defend, not to PTH itself.

PTH only makes sense as part of a small cast. It sits with calcium, phosphorus, and vitamin D on a bone and mineral panel, each read against the others. The metabolic panel guide covers how calcium first lands on routine bloodwork, which is often what prompts a PTH at all.

A single paired snapshot can still miss a slow problem that a closer reading would catch. A PTH creeping up while calcium drifts toward the top of its range tells a story neither value tells alone, and even finding the result can mean decoding the printout, since most labs list it as intact PTH or just the code PTH; the guide to abbreviations on lab reports helps you match the shorthand to the marker. The pump that should switch itself off is worth watching when it keeps running.