What is the parathyroid hormone?
- 84 amino acid polypeptidehormone
- synthesized, stored, and secreted
- Regulates calcium and phosphorus homeostasis
Describe the negative feedback loop of the parathyroid hormone
Increased blood calcium:
- Parathyroid hormone is released from thyroid
- increase deposition of calcium in the bone matrix
Decreased blood calcium:
- Releases PTH
- promotes release of calcium from bone
- Encourages re-absorption of calcium and magnesium by kidneys
- inhibits re-absorption of phosphates in the kidneys
- kidneys release calcitrol affecting GI absorption
Primary Hyperparathyroidism
- Abnormal glands that secrete too much PTH
- Causes: Adnenomas (85%), hyperplasia (10%), ectopic <5%, and parathyroid cancer (<1%)
- Hyperplasia: Enlargement of all 4 glands, 20% recurrence
What is secondary hyperparathyroidism?
- A condition caused by a disease outside the PT glands causing PT glands to become enlarged and hyperactive
- Surgery is not indicated
1. low serum Ca2+ and elevated phosphoruc
2. Ex: renal insufficiency, vit D deficiency
3. Stimulation of PTHis a physiological mechanism that occurs in patients with renal insufficiency
4. Despite elevated PTH, serum Ca2 + remains low
What is tertiary hyperparathyroidism?
- renal failure and autonomous functioning gland
- presents as hypercalcemia in patients in with renal failure
- 1 or more parathyroid glands becomes autonomous and no longer respinds to PTH
- Surgical resection
Single parathyroid adenoma
- one adenoma
- 85% of people
Multiple parathyroid adenomas
- adenomas in 2 or 3 ofthe parathyroid glands
- 10% of people
Parathyroid hyperplasia
-5% of people
- all 4 glands are overactive
Parathyroid cancer
- less than 0.1% of people with hyperparathyroidism
Symptoms of hyperparathyroidism
- historically symptomatic, oftem asymptomatic
- often an incidental finding
- Dx with routine bloodwork: increased serum calcium, decreased serum phosphate
- PTH level ordered after positive bloodwork results
Indications for a parathyroid study
- Localization, not diagnosis
1. decreses invasive surgery
2. intra-operative gamma probes
3. especially for repeat operations - Differentiation from hyperplasia vs. adenomas
- localization of parathyroid tissue after Sx for persistent or recurrecnt HPT
- Surgical treatment
1. prevents mineral or bone loss
2. hypersplasia 3.5 of 4 glands
3. adenoma affected gland(s)
Contraindications
- Pregnancy
- breastfeeding: precautions
- cant be immobilized
- recent NM study
- patient not prepped
Patient preparation
- Can eat, drink, and take meds
- Serum Ca and PTH measured within 1 month of exams
- Patient history: clinical lab results, surgery, recent imaging
- Hx of thyroid disease
- Recent IV contrast or thyroid hormone of doing thyroid subtraction method
Energy, half life, localization, dose, and administration
99m-Tc Sestamibi
- 140 Kev
- T 1/2: 6 hours
- localization vascularity and mitochondria in oxyphils- slow release from cells
- 740-1110 MBq
- IV injection
Energy, half life, localization, dose, special prep
123-I NaI
- 159 Kev
- T 1/2: 13 hours
- Organification
- 7.5-22 MBq
- Prep: Anti thyroid meals, dietary iodine, oral/IV contrast
collimator, matrix, positions, fov, times
Planar imaging protocol
- LEHR or pinhole
- 256 x 256 or 128 x 128
- Patient supine
- Anterior mediastinum and neck
- Wait 10-30 min post- 99m-Tc mibi
- Wait 4 hrs post 123-I
- 10 min
- Zoom 1.5
SPECT Imaging Protocol
- LEHR
- 128 x 128
- Patient supine
- Anterior mediastinum and neck
- 360 degree
- 15-25 seconds per stop
- CT for attenuation correction
Low Dose CT parameters
- kV: 120
- mA: 20
- Reconstruction: Iterative
- Matrix: 512 x 512
- Head-first
- 3 segments
- Breath hold
Technical considerations
- Adenoma: Obliques, pinhole
- patient movement
- image misalignment
- substraction artifacts
Normal results
- thyroid washout
- non visualization of parathyroid glands
Abnormal Results
- > 300g
- 90% sensitivity
False negative results
- adenoma results
- rapid washout
False positive
- thyroid follicular adenoma
- thyroid cancer
- metastasis
Describe Mibi and pertechnetate subtraction
- Pertechnetate uptake in the thyroid
- can be subtracted by the parathyroid
- Remaining tissue is parathyroid
- Mibi uptake in the parathyroid
