Thyroid, Parathyroid, and Calcium Disorders

1. Normal function first

The thyroid, in the front of the neck, makes two iodine-containing hormones: thyroxine (T4, the more abundant storage form) and triiodothyronine (T3, the more active form, largely converted from T4 in tissues). The hypothalamic–pituitary–thyroid axis controls them: the hypothalamus releases thyrotropin-releasing hormone (TRH), which prompts the pituitary to release thyroid-stimulating hormone (TSH), which drives the thyroid to make and release T3 and T4. Rising thyroid hormone feeds back to suppress TSH and TRH.

Thyroid hormones raise the basal metabolic rate—increasing oxygen consumption and heat production, heart rate and contractility, and supporting growth, nervous-system development, and the action of other hormones.

Calcium balance is managed separately by the four parathyroid glands behind the thyroid. When blood calcium falls, they release parathyroid hormone (PTH), which raises calcium three ways: stimulating bone resorption (releasing calcium), increasing kidney calcium reabsorption while excreting phosphate, and, via kidney-activated vitamin D, increasing gut calcium absorption. This keeps blood calcium in a narrow range needed for nerve signaling, muscle contraction, clotting, and bone strength.

2. What changes in disease

Hypothyroidism is thyroid hormone deficiency, most often from autoimmune destruction (Hashimoto thyroiditis) or iodine deficiency. With too little T3/T4, metabolism slows: fatigue, cold intolerance, weight gain, constipation, slow heart rate, dry skin, and slowed thinking can develop; because feedback is intact, TSH is typically high (the pituitary keeps "pressing the pedal"). Hyperthyroidism is excess thyroid hormone, most often from Graves disease (autoimmune stimulation). It produces weight loss, heat intolerance, fast heart rate, tremor, anxiety, and increased bowel activity, with a low TSH because feedback suppresses it.

Goiter is thyroid enlargement, arising from iodine deficiency (the gland enlarges trying to trap iodine), autoimmune stimulation, inflammation, or nodules; it can occur in hypo- or hyperthyroid states, so size alone does not indicate hormone level. Thyroiditis is thyroid inflammation—autoimmune (Hashimoto, often leading to hypothyroidism) or post-viral (subacute)—and can release stored hormone transiently before a hypothyroid phase.

Parathyroid disorders alter calcium. Hypoparathyroidism is deficient PTH (commonly after neck surgery), producing hypocalcemia; because calcium stabilizes nerve membranes, low calcium causes neuromuscular irritability—tingling, cramps, and, in severe cases, tetany. Hyperparathyroidism is excess PTH, usually from a benign adenoma (primary) or from chronic kidney disease or vitamin D deficiency (secondary). Excess PTH causes hypercalcemia and, over time, bone demineralization, kidney stones, and GI symptoms such as nausea and constipation.

3. Why the changes matter

These disorders are widespread and often subtle at first. Hypothyroidism may be mistaken for aging or depression; hyperthyroidism can strain the cardiovascular system (arrhythmias, especially in older adults). Calcium disturbances affect the nervous system, heart rhythm, bones, and kidneys. Lab patterns—high TSH with low thyroid hormone in hypothyroidism, low TSH with high thyroid hormone in hyperthyroidism, and paired calcium/PTH measurements in parathyroid disease—reveal whether the problem lies in the gland itself or in the feedback loop.

How it works

1. Blood calcium falls, and the parathyroid glands detect the drop.
2. PTH release increases.
3. PTH acts on three targets: bone releases calcium, the kidney retains calcium and excretes phosphate, and the kidney activates vitamin D to boost gut calcium absorption.
4. Blood calcium rises, and PTH release is reduced by negative feedback, restoring balance.

Common confusions

Memory aids

Remember the four PTH calcium targets as "B-K-G": Bone, Kidney, Gut. For thyroid labs, recall "If the gland is lazy, TSH is crazy-high"—primary hypothyroidism raises TSH because the pituitary keeps signaling a failing gland.

Quick review

Topic Recap

• Thyroid hormone (T3/T4), driven by TSH, sets the metabolic rate and affects nearly every organ.
• Hypothyroidism slows the body and raises TSH; hyperthyroidism speeds it and lowers TSH (in primary disease).
• Goiter indicates gland enlargement, and thyroiditis indicates inflammation—both can shift hormone levels over time.
• PTH and vitamin D raise blood calcium through bone, kidney, and gut actions.
• Hypoparathyroidism lowers calcium (neuromuscular irritability); hyperparathyroidism raises it (bone loss, stones).

Knowledge Check

1. In primary hypothyroidism, is TSH high or low, and why?
2. Which two autoimmune diseases are common causes of hypo- and hyperthyroidism, respectively?
3. Why can a goiter be present in both hypothyroid and hyperthyroid states?
4. What are the three target organs through which PTH raises blood calcium?
5. How does hypoparathyroidism produce muscle cramps and tetany?

Answers and Rationales

1. Answer: High—because feedback is intact, the pituitary increases TSH to stimulate a failing thyroid. Why: A high TSH with low thyroid hormone points to a primary (gland-level) problem.
2. Answer: Hashimoto thyroiditis (hypothyroidism) and Graves disease (hyperthyroidism). Why: Both are autoimmune, but Hashimoto destroys the gland while Graves stimulates it.
3. Answer: Goiter reflects gland size, which can enlarge from iodine deficiency, autoimmune stimulation, inflammation, or nodules—none of which alone defines hormone output. Why: Size and function are separate dimensions.
4. Answer: Bone (resorption releases calcium), kidney (retains calcium, excretes phosphate), and gut (via vitamin D–enhanced absorption). Why: Together these three actions raise blood calcium.
5. Answer: Low PTH drops blood calcium, and calcium stabilizes nerve membranes, so low calcium makes nerves and muscles more excitable—causing tingling, cramps, and tetany. Why: Hypocalcemia lowers the threshold for neuromuscular firing.