Hyperthyroidism | Clinical Medicine

Hyperthyroidism is characterized by an overactive thyroid gland. It can be primary (thyroid gland problem) or secondary (hypothalamus or pituitary problem). Primary hyperthyroidism involves the thyroid gland pumping out excessive T3 and T4, which can happen through hyperfunctioning thyroid follicles or destructive processes. The latter leads to a transient hyperthyroidism, while hyperfunctioning is often permanent.

In primary hyperthyroidism, high T3/T4 levels inhibit the hypothalamus and pituitary, resulting in low TSH. Another cause can be exogenous thyroid hormone intake (iatrogenic).

Graves' disease is the most common cause of primary hyperthyroidism, characterized by autoantibodies (TSH receptor antibodies) stimulating TSH receptors diffusely across the thyroid. Other causes include toxic adenoma (a single benign mass with hyperactive TSH receptors) and toxic multinodular goiter (multiple benign hyperactive masses). The Jod-Basedow phenomenon describes hyperthyroidism triggered by iodine intake in patients with existing thyroid conditions like Graves' or toxic nodular goiter.

Thyroid destruction leads to transient hyperthyroidism. Hashimoto's thyroiditis, initially, can cause a brief hyperthyroid state due to follicle destruction and release of stored hormones, eventually leading to chronic hypothyroidism. Postpartum thyroiditis and de Quervain's thyroiditis (subacute granulomatous thyroiditis, often post-viral and painful) also cause transient hyperthyroidism followed by a hypothyroid state and eventual recovery (triphasic response), with de Quervain's distinguished by a very painful thyroid and high ESR.

Secondary hyperthyroidism results from a pituitary problem, usually a microadenoma, which causes high TSH and subsequent high T3/T4 levels. This is a key diagnostic differentiator from primary hyperthyroidism (low TSH, high T3/T4). Pituitary microadenomas, unlike macroadenomas, are less likely to cause mass effect but can secrete other hormones, leading to associated conditions like gynecomastia/galactorrhea (high prolactin), Cushing's disease (high ACTH), and acromegaly (high growth hormone).

Common findings include a goiter (due to TSH or TSH receptor antibody stimulation, especially in Graves' and secondary hyperthyroidism), metabolic changes (increased metabolic rate, leading to weight loss, increased hunger, and heat intolerance due to increased sodium-potassium ATPase activity), and neurological effects (hyperactive behavior like agitation, irritability, anxiety, insomnia, tremors, and hyperactive deep tendon reflexes).

Cardiovascular effects include increased heart rate (tachycardia, often resulting in atrial fibrillation or SVT) and increased contractility, leading to increased cardiac output and systolic hypertension. Hyperthyroidism also affects reproductive function, increasing thyroxine-binding globulin, which binds to estrogen, progesterone, and testosterone, leading to reduced free levels. This can cause reduced libido, infertility, and menstrual abnormalities.

Gastrointestinal dysfunction manifests as increased GI motility, leading to diarrhea. Integumentary system manifestations are particularly important in Graves' disease due to TSH receptor antibodies affecting fibroblasts and adipose tissue. This leads to increased glycosaminoglycans and adipocyte formation, causing pretibial myxedema (non-pitting edema in lower extremities) and exophthalmos (protruding eyeballs due to retro-orbital adipose tissue and fluid accumulation), both characteristic of Graves' opthalmopathy.

Thyroid storm is a life-threatening complication of chronic hyperthyroidism combined with a massive stressor (e.g., infection, surgery, iodine load, or non-compliance with anti-thyroid drugs), which amplifies sympathetic nervous system activity and thyroid hormone levels. This leads to extreme hyperthermia (>41.5°C), severe agitated delirium, profound tachycardia (often refractory arrhythmias), and high-output cardiac failure. Despite increased cardiac output, the metabolic demands of the tissues are not met.

Diagnosis begins with Thyroid Function Tests (TFTs). High T4 + high TSH suggests secondary hyperthyroidism (pituitary microadenoma, confirm with pituitary MRI). High T4 + low TSH suggests primary hyperthyroidism. To differentiate causes of primary hyperthyroidism: if Graves' is suspected (goiter, exophthalmos, pretibial myxedema), test for TSH receptor antibodies. If not clear, perform a radioactive iodine uptake scan: focal uptake (toxic adenoma), diffuse uptake (Graves'), no uptake (thyroiditis), or diffuse/low uptake combined with low thyroglobulin (iatrogenic from exogenous thyroid hormone).

Initial treatment for hyperthyroidism involves managing symptoms like tachycardia, tremors, anxiety with propranolol (a beta-blocker) and suppressing thyroid hormone production with anti-thyroid drugs like PTU (propylthiouracil) or methimazole. PTU is often preferred in the first trimester of pregnancy due to methimazole's teratogenic effects. If these are ineffective, radioactive iodine ablation to destroy thyroid follicles or surgical thyroidectomy can be considered, especially for large goiters.

Thyroid storm is an emergency. Treatment includes cooling measures for hyperthermia, propranolol for tachycardia, and a specific sequence of medications: PTU (preferred over methimazole due to its ability to inhibit T4 to T3 conversion) to block thyroid hormone synthesis, followed by potassium iodide (Lugol's solution) an hour later to further block hormone release (Wolff-Chaikoff effect), and then hydrocortisone to suppress T4 to T3 conversion, thereby reducing the active form of the hormone. This multi-pronged approach tackles hormone production, release, and peripheral conversion.