The thyroid and parathyroid glands are located in the neck but perform very different functions. The thyroid primarily regulates metabolism, energy use, temperature regulation, heart rate, and growth, while the parathyroid glands primarily regulate calcium and phosphorus levels in the body.

Thyroid Gland 
 

The Thyroid gland produces the hormones T4 (thyroxine) and T3 (triiodothyronine). Most thyroid hormone released by the thyroid is T4, which is converted into the more active T3 in tissues throughout the body.

The pituitary gland regulates thyroid hormone production by releasing thyroid-stimulating hormone (TSH). The American Thyroid Association recommends screening adults for thyroid dysfunction with a TSH beginning at age 35 and every 5 years thereafter, with more frequent testing when symptoms or risk factors are present.

Diagnostics

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Subclinical hypothyroidism = ↑ TSH, Normal Free T4

Overt hypothyroidism = ↑ TSH, ↓ Free T4

Subclinical hyperthyroidism = ↓ TSH, Normal Free T4

Overt hyperthyroidism = ↓ TSH, ↑ Free T4 (and/or T3 ↑) 

Central hypothyroidism = Normal or ↓ (sometimes mildly ↑) TSH, ↓ Free T4

 

Hypothyroidism
 

Hypothyroidism occurs when the body does not have enough thyroid hormone. The most common cause in the United States is Hashimoto's thyroiditis, an autoimmune condition in which the immune system gradually damages the thyroid.

 

Other causes include thyroid surgery, radioactive iodine treatment, certain medications, congenital thyroid disorders, pituitary disease, and severe iodine deficiency or excess.

 

Possible symptoms include:
 

• Fatigue or excessive sleepiness

• Feeling unusually cold

• Weight gain

• Constipation

• Dry skin

• Hair thinning or hair loss

• Muscle aches, cramps, or weakness

• Slower heart rate

• Depression or difficulty concentrating

• Menstrual changes

• Fertility problems

• Puffy face or swelling

• Hoarse voice

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Testing may include:
 

• TSH – usually the initial screening test

• Free T4 – helps determine the amount of available thyroid hormone

• Thyroid peroxidase (TPO) antibodies – may help identify Hashimoto's thyroiditis

 

Most cases of primary hypothyroidism show high TSH and low free T4.

 

Treatment: Hypothyroidism is usually treated with levothyroxine, a synthetic form of T4. Blood testing is repeated after starting or changing the dose to ensure thyroid hormone levels are appropriate.

 

Hyperthyroidism

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Hyperthyroidism occurs when the thyroid produces excessive thyroid hormone. Graves' disease, an autoimmune disorder that stimulates the thyroid, is a common cause.

Other causes include toxic thyroid nodules, toxic multinodular goiter, thyroiditis, excessive thyroid hormone medication, and certain medications or iodine exposures.

 

Possible symptoms include:

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• Unexplained weight loss

• Rapid or irregular heartbeat

• Palpitations

• Tremor

• Anxiety, irritability, or restlessness

• Heat intolerance

• Excessive sweating

• Frequent bowel movements

• Muscle weakness

• Difficulty sleeping

• Menstrual changes

• Enlarged thyroid (goiter)

 

Graves' disease may also cause eye symptoms, including eye irritation, pressure, bulging, double vision, or eyelid changes.

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Testing may include:

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• TSH

• Free T4

• T3

• TSH-receptor antibodies (TRAb) or thyroid-stimulating immunoglobulin (TSI) when Graves' disease is suspected

• Radioactive iodine uptake/thyroid scan in selected patients

• Thyroid ultrasound when structural abnormalities or nodules need evaluation

 

Most cases of primary hyperthyroidism show low TSH with elevated free T4 and/or T3.

 

Treatment depends on the cause and may include:
 

• Beta blockers to control symptoms such as tremor and rapid heart rate

• Antithyroid medication, most commonly methimazole

• Radioactive iodine

• Thyroid surgery

 

Thyroid Nodules and Goiter

 

A thyroid nodule is a lump within the thyroid gland. Nodules are common, and the large majority are benign. A goiter simply means an enlarged thyroid. The thyroid may be enlarged because of autoimmune thyroid disease, nodules, iodine abnormalities, or other conditions. Symptoms such as progressive difficulty swallowing, difficulty breathing, persistent hoarseness, rapidly enlarging neck mass, or enlarged neck lymph nodes require further evaluation.

 

Evaluation may include:
 

• TSH

• Thyroid ultrasound

• Additional thyroid hormone testing when indicated

• Radioactive iodine scan when TSH is suppressed

• Fine-needle aspiration (FNA) biopsy for nodules meeting certain ultrasound and size criteria

*Ultrasound is primarily used to evaluate thyroid enlargement or nodules, while radioactive iodine uptake/thyroid scanning may be used when TSH is suppressed to determine the cause of hyperthyroidism and whether a nodule is

hyperfunctioning ("hot") or nonfunctioning ("cold").
 

• Hot nodule: producing thyroid hormone autonomously → malignancy risk is low, and FNA generally isn't performed on the hyperfunctioning nodule.

• Cold/nonfunctioning nodule: assess according to its ultrasound characteristics and size → FNA may be indicated.

 

Parathyroid Glands

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Most people have four parathyroid glands located behind the thyroid. Despite their name and location, they perform a different function from the thyroid.

The parathyroid glands produce parathyroid hormone (PTH), which helps maintain an appropriate calcium level in the blood.

When blood calcium falls, PTH helps increase it by influencing the bones, kidneys, and vitamin D activation, which in turn affects calcium absorption from the intestines.

 

Hyperparathyroidism

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Hyperparathyroidism occurs when excessive PTH is produced.

 

Primary Hyperparathyroidism
 

Primary hyperparathyroidism occurs when one or more parathyroid glands independently produce too much PTH. A benign parathyroid adenoma is the most common cause.

 

Typical laboratory findings include:
 

High calcium + elevated or inappropriately normal PTH

An elevated calcium level should normally suppress PTH. Therefore, a PTH result that falls within the laboratory's "normal" range can still be abnormal when calcium is elevated.

Many people have no obvious symptoms and are diagnosed through routine blood testing.
 

When symptoms or complications occur, they may include:
 

• Kidney stones

• Increased urination and thirst

• Constipation

• Abdominal discomfort

• Bone loss or osteoporosis

• Muscle weakness

• Fatigue

• Difficulty concentrating or mood changes

Evaluation may include:
 

• Total calcium and albumin and/or ionized calcium

• PTH

• Creatinine/eGFR

• Phosphorus

• Vitamin D

• 24-hour urine calcium in selected patients

• Bone-density testing

• Kidney imaging when stones are suspected

Treatment: Some patients can be monitored, while others benefit from surgical removal of the abnormal parathyroid gland(s), particularly when specific surgical criteria are met.

Secondary Hyperparathyroidism
 

Secondary hyperparathyroidism occurs when another condition chronically stimulates the parathyroid glands.

Common causes include:

• Chronic kidney disease

• Vitamin D deficiency

• Low calcium intake or impaired calcium absorption

Unlike primary hyperparathyroidism, the calcium level is usually normal or low rather than elevated.

Treatment focuses primarily on correcting the underlying cause.

Hypoparathyroidism
 

Hypoparathyroidism occurs when insufficient PTH is produced, resulting in low blood calcium. It most commonly develops after thyroid or neck surgery that damages or removes the parathyroid glands, although autoimmune and genetic causes can also occur.
 

Possible symptoms of low calcium include:

• Tingling around the mouth

• Tingling or numbness of the fingers or toes

• Muscle cramps

• Muscle spasms

• Twitching

• Weakness

• Seizures in severe cases

• Abnormal heart rhythms in severe cases

Testing may show:
 

Low calcium + low or inappropriately normal PTH

Phosphorus is often elevated.

Treatment may include calcium and active vitamin D (calcitriol). Some patients with chronic hypoparathyroidism may require additional specialized treatment.

When Calcium Is Abnormal
 

An abnormal calcium result does not automatically mean a parathyroid disorder. Calcium abnormalities can occur from dehydration, kidney disease, vitamin D disorders, medications, malignancy, abnormal protein levels, and other conditions.

Because much of the calcium in blood is attached to albumin, an abnormal total calcium result may sometimes require evaluation with albumin or ionized calcium before determining whether a true calcium abnormality is present.

testing when appropriate. 

Low albumin → total calcium may look low even though ionized calcium is normal.

This is sometimes called pseudohypocalcemia.

Historically, clinicians have calculated a "corrected calcium":

Corrected Ca ≈ measured Ca + 0.8 × (4 − albumin)

However, albumin-correction formulas are imperfect. If knowing the true calcium level matters clinically, directly measuring ionized calcium is preferable. Low albumin → less circulating bound Ca → lower total Ca, while ionized Ca can remain normal.

When would you order ionized calcium?
 

You don't need an ionized calcium on everyone. It's particularly useful when:
 

• Total calcium is abnormal but you're unsure whether it represents a true calcium abnormality.

• Albumin is substantially abnormal.

• Total calcium and the clinical picture don't agree.

• You're evaluating suspected hyperparathyroidism and the total calcium is borderline or intermittently elevated.

• You're considering normocalcemic primary hyperparathyroidism.

• The patient is critically ill.

• There are major acid-base abnormalities, because pH changes calcium binding to albumin.

• The patient has symptoms suggesting hypocalcemia despite a seemingly normal total calcium.