Hypothyroidism

Hypothyroidism is the clinical state that results from insufficient thyroid hormone action, most often because the thyroid gland itself fails to produce enough hormone (primary hypothyroidism). Because thyroid hormone supports the metabolic rate of nearly every tissue, deficiency produces a broad, often nonspecific slowing of bodily functions, and the diagnosis rests on biochemical testing rather than symptoms alone.

Definition

Hypothyroidism is a condition of deficient thyroid hormone action, usually from failure of the thyroid gland to secrete adequate thyroxine and triiodothyronine (primary hypothyroidism), characterized biochemically by an elevated thyroid-stimulating hormone with low or low-normal free thyroxine, and classified under thyroid diseases.

Scope

This entry covers primary and central hypothyroidism, the distinction between overt and subclinical disease, the principal causes (autoimmune thyroiditis and iodine deficiency foremost), the systemic consequences of hormone deficiency, and the epidemiology of the condition. It treats hypothyroidism as a reference clinical entity and does not provide diagnostic thresholds, dosing, or individualized management.

Core questions

• What distinguishes primary hypothyroidism (thyroid gland failure) from central hypothyroidism (pituitary or hypothalamic causes)?
• How are overt and subclinical hypothyroidism defined, and why does the distinction matter?
• Why does thyroid hormone deficiency produce such broad, nonspecific systemic effects?

Key concepts

• Primary versus central hypothyroidism
• Overt versus subclinical hypothyroidism
• Elevated TSH with negative feedback
• Autoimmune (Hashimoto) thyroiditis as leading cause
• Iodine deficiency
• Congenital hypothyroidism
• Myxedema
• Levothyroxine replacement (concept)

Mechanisms

In primary hypothyroidism the thyroid cannot maintain normal hormone output, so circulating T4 and T3 fall and the pituitary, released from feedback inhibition, raises TSH; the elevated TSH with low free T4 is the biochemical signature of overt disease, while a raised TSH with still-normal free T4 defines the subclinical state (Chaker, 2017). The deficiency slows metabolic processes across tissues, producing fatigue, cold intolerance, weight gain, slowed cognition, and, in severe long-standing cases, the mucinous tissue changes termed myxedema (Chaker, 2017). Central hypothyroidism, by contrast, arises from inadequate TSH drive due to pituitary or hypothalamic disease, so TSH is not appropriately elevated despite low hormone. Management is conceptually replacement of the missing hormone, usually with levothyroxine, whose pharmacology rests on peripheral conversion of T4 to active T3 (Biondi, 2014; Jonklaas, 2014).

Clinical relevance

Hypothyroidism is one of the most common endocrine diagnoses and its nonspecific symptoms mean it is considered across many clinical settings; it is also a recognized contributor to dyslipidemia and, in pregnancy, to maternal and fetal risks. This entry supports critical appraisal of the evidence and guidelines on how hypothyroidism is defined and studied; it is educational reference material and not a basis for individual diagnostic or treatment decisions (Jonklaas, 2014).

Epidemiology

Hypothyroidism is common and more frequent than hyperthyroidism, with prevalence rising with age and a strong female preponderance; population burden is shaped by iodine status, being higher where iodine intake is deficient, and autoimmune thyroiditis is the dominant cause in iodine-sufficient populations (Taylor, 2018; Vanderpump, 2011).

Debates

When, if ever, should subclinical hypothyroidism be treated?

Whether a raised TSH with normal free T4 warrants thyroid hormone, and at what threshold, is debated because the symptomatic and cardiovascular benefits are uncertain and vary with age and degree of TSH elevation.

Is levothyroxine monotherapy sufficient for all patients?

Some patients on adequate T4 replacement report persistent symptoms, prompting discussion of whether combined T4/T3 therapy helps a subgroup; the evidence remains inconclusive.

Seminal works

• chaker-2017
• jonklaas-2014
• taylor-2018

Frequently asked questions

What is the difference between overt and subclinical hypothyroidism?

Overt hypothyroidism has both an elevated TSH and a low free thyroxine, whereas subclinical hypothyroidism has an elevated TSH but a free thyroxine still within the normal range, representing a milder or earlier biochemical state.

What is the most common cause of hypothyroidism?

In iodine-sufficient populations the leading cause is chronic autoimmune (Hashimoto) thyroiditis, while worldwide iodine deficiency remains a major cause; the relative importance depends on population iodine status.