Thyroid & calcium disorders — Hypo- and hyperthyroidism, thyroid emergencies, and disorders of calcium and PTH.

The understanding of thyroid and calcium physiology has evolved over centuries, transforming what were once mysterious wasting diseases and skeletal deformities into treatable endocrine conditions. Before the identification of thyroid hormones, clinicians in the alpine regions of Europe documented endemic goiter and cretinism without understanding their shared etiology of iodine deficiency. Similarly, the observation that parathyroid gland removal during thyroidectomy produced fatal tetany led to the discovery of parathyroid hormone (PTH) and its central role in calcium homeostasis. These milestones form the foundation for the clinical syndromes tested on the USMLE Step 2 examination.

Today, thyroid and calcium disorders remain among the most commonly tested endocrine topics on USMLE Step 2. The central clinical questions are straightforward: How do you distinguish hypo- from hyperthyroidism using the TSH–free T₄ axis? When does thyroid disease become an emergency? And how do you localize the source of a calcium derangement using the PTH level? The sections that follow build a systematic framework for answering each of these questions.

Thyroid and calcium physiology share a conceptual architecture built on negative feedback loops and hormone–receptor interactions at distant target organs. Understanding five foundational principles enables you to interpret nearly every clinical scenario involving these axes.

The diagram above encapsulates the single most important diagnostic reflex in thyroid medicine. When you suspect thyroid dysfunction, the first laboratory value to order is serum TSH. Because of the log-linear relationship between TSH and free T₄, even small changes in circulating thyroid hormone produce large, amplified shifts in TSH. A high TSH immediately directs you toward hypothyroidism; a low TSH toward hyperthyroidism. Free T₄ is then measured to determine severity — subclinical disease shows an abnormal TSH but a normal free T₄, whereas overt disease shows derangement of both. The rare exceptions involve central pathology (pituitary or hypothalamic disease), where the TSH is 'inappropriately normal' or low in the setting of low free T₄ — a pattern that should trigger pituitary MRI and further workup.

Hypothyroidism

Primary hypothyroidism accounts for over 95% of cases and results from destruction or dysfunction of the thyroid gland itself. In iodine-sufficient regions, Hashimoto thyroiditis — chronic lymphocytic thyroiditis with anti-thyroid peroxidase (anti-TPO) and anti-thyroglobulin antibodies — is the most common etiology. Other causes include post-radioactive iodine ablation, post-surgical hypothyroidism, medications (amiodarone, lithium, checkpoint inhibitors), and radiation exposure. The deficiency of T₃ slows cellular metabolism throughout the body: patients present with fatigue, cold intolerance, weight gain, constipation, dry skin, bradycardia, and delayed deep tendon reflexes. Laboratory findings classically show elevated TSH, low free T₄, elevated LDL cholesterol, hyponatremia (from impaired free water excretion), and macrocytic anemia. Treatment is levothyroxine (T₄) replacement, dosed at approximately 1.6 µg/kg/day, taken on an empty stomach, with TSH monitored every 6–8 weeks until stable.

Hyperthyroidism

Hyperthyroidism is defined by excess circulating thyroid hormone with suppressed TSH. Graves disease — caused by thyroid-stimulating immunoglobulin (TSI) that activates the TSH receptor — is the most common cause in younger patients and the only cause associated with ophthalmopathy and pretibial myxedema. Toxic multinodular goiter and toxic adenoma are autonomously functioning nodular diseases more common in elderly patients. Subacute (de Quervain) thyroiditis causes a transient hyperthyroid phase from thyroid destruction with subsequent hypothyroidism. A critical differentiating tool is the radioactive iodine uptake (RAIU) scan: diffusely increased uptake suggests Graves disease, focal hot nodule(s) suggest toxic adenoma or toxic multinodular goiter, and low uptake suggests thyroiditis or exogenous thyroid hormone ingestion. Clinical features reflect a hypermetabolic state — heat intolerance, weight loss, anxiety, tremor, tachycardia, atrial fibrillation, and hyperdefecation. Management options include thionamides (methimazole preferred, propylthiouracil in first trimester pregnancy), radioactive iodine ablation, and thyroidectomy. Beta-blockers (propranolol) control adrenergic symptoms and additionally inhibit peripheral T₄-to-T₃ conversion.

Thyroid Emergencies

Thyroid storm represents the extreme decompensation of hyperthyroidism, typically precipitated by infection, surgery, trauma, or iodinated contrast in a patient with uncontrolled Graves disease. Clinical features include high fever (>40°C), tachycardia out of proportion, altered mental status, and cardiovascular collapse. The Burch–Wartofsky score helps quantify clinical suspicion but treatment should not be delayed awaiting labs. Management follows a specific sequence: (1) propylthiouracil (PTU) to block new hormone synthesis and peripheral T₄→T₃ conversion, (2) iodine solution (SSKI or Lugol's) given at least one hour after PTU to block thyroid hormone release via the Wolff–Chaikoff effect, (3) beta-blocker for rate control, (4) hydrocortisone to prevent relative adrenal insufficiency and further block peripheral conversion, and (5) supportive care with cooling and volume resuscitation.

Myxedema coma is the life-threatening decompensation of severe hypothyroidism, often triggered by infection, cold exposure, or sedative use in an elderly patient with longstanding untreated hypothyroidism. Key features include hypothermia, altered mental status, bradycardia, hypoventilation, and hyponatremia. Treatment requires intravenous levothyroxine (or liothyronine), intravenous hydrocortisone (given empirically before thyroid hormone to avoid precipitating adrenal crisis), passive rewarming, and supportive ICU care.

Calcium homeostasis is maintained through the coordinated actions of PTH, vitamin D, and calcitonin acting on bone, kidney, and intestine. The diagnostic approach to calcium disorders centers on two questions: Is the calcium truly abnormal (always correct for albumin or use ionized calcium)? And is PTH elevated or suppressed? The relationship between calcium and PTH defines the major diagnostic categories and guides the clinician toward the underlying etiology.

Primary Hyperparathyroidism

Primary hyperparathyroidism (PHPT) is the most common cause of hypercalcemia in the outpatient setting. Approximately 85% of cases are caused by a single parathyroid adenoma, with the remainder due to four-gland hyperplasia (often in MEN syndromes) or, rarely, parathyroid carcinoma. Many patients are now detected incidentally on routine chemistry panels, presenting with asymptomatic hypercalcemia. When symptomatic, the classic mnemonic — stones (nephrolithiasis), bones (osteoporosis, osteitis fibrosa cystica), groans (abdominal pain, constipation, pancreatitis), and psychiatric overtones (depression, confusion) — captures the spectrum. Diagnostic labs show elevated calcium, elevated or inappropriately normal PTH, low phosphorus, and elevated 24-hour urine calcium (the latter distinguishing PHPT from familial hypocalciuric hypercalcemia, where urine calcium is low). Definitive treatment is parathyroidectomy, guided by preoperative sestamibi scan or 4D-CT for localization.

Hypercalcemia of Malignancy

Malignancy is the most common cause of hypercalcemia in the inpatient setting. Mechanisms include humoral hypercalcemia via PTH-related peptide (PTHrP) secretion (squamous cell carcinomas of the lung, renal cell carcinoma), osteolytic metastases with local cytokine release (breast cancer, multiple myeloma), and excess calcitriol production (lymphomas). PTH is appropriately suppressed in all malignancy-related hypercalcemia. Acute management follows the sequence of aggressive IV normal saline hydration → calcitonin for rapid but short-lived effect → IV bisphosphonate (zoledronic acid or pamidronate) for sustained calcium lowering, which takes 2–4 days to reach full effect. Denosumab is an option for bisphosphonate-refractory cases.

Hypocalcemia

The most common causes of hypocalcemia include hypoparathyroidism (post-surgical is the most frequent etiology, followed by autoimmune destruction), vitamin D deficiency, chronic kidney disease (reduced 1-alpha-hydroxylation), and hypomagnesemia — a critical and frequently tested cause that impairs both PTH secretion and PTH receptor signaling. Clinical features of hypocalcemia include neuromuscular irritability (tetany, carpopedal spasm), Chvostek sign (facial twitch on tapping the facial nerve) and Trousseau sign (carpal spasm with blood pressure cuff inflation), perioral numbness, seizures, prolonged QT interval, and in severe cases, laryngospasm. Pseudohypoparathyroidism (Albright hereditary osteodystrophy) represents end-organ resistance to PTH, producing low calcium, high phosphorus, and elevated PTH with characteristic phenotypic features (short stature, short fourth metacarpals, round facies).

Thyroid and calcium disorders frequently intersect with other clinical domains tested on Step 2, and certain special populations require modified management approaches that are high-yield for examinations.

These advanced topics are frequently tested in integrated clinical vignettes where thyroid or calcium pathology coexists with pregnancy, renal disease, or genetic syndromes. The unifying principle remains the same: anchor your reasoning in the feedback loop architecture of the HPT axis and the PTH–calcium axis. Even complex scenarios reduce to asking whether the feedback loop is intact or disrupted, and at which level the disruption occurs — gland, receptor, or end-organ.