Chapter 122: Thyroid Hormones

High-Yield Facts

• Acute ingestions are generally benign but may be delayed in onset (2–4 days).

• Ingestions of greater than 5 mg warrant measurement of T4 concentration at about 4 hours after ingestion. A T4 level of greater than 75 mcg/dL warrants close follow-up such as daily assessment for signs and symptoms of thyroid hormone excess.

• Symptoms are usually mild and consist of adrenergic excess: tachycardia, hypertension, fever.

• Treatment is symptomatic and may include beta-blocking agents and antipyretics.

Introduction

As with other hormones of the endocrine system, the thyroid hormones play an essential role in the regulation of metabolic processes. Working through an elegant feedback mechanism between the thyroid, pituitary gland, hypothalamus, and the target tissues, they help regulate cardiovascular function, temperature control, and metabolic rate. Exogenous hormone administration is used as replacement therapy in hypothyroidism (either naturally occurring or post-ablation treatment for uncontrollable hyperthyroidism) or as suppressive therapy in cases of thyroid carcinoma. Thyroid hormones may also be abused by patients for weight control or for other psychological reasons. This syndrome has been termed thyrotoxicosis factitia and often involves medical professionals.¹ Because of the feedback mechanisms, acute ingestion of thyroid hormones is generally benign, even in amounts significantly greater than those used therapeutically. In 2011, the AAPCC reported 9457 exposures to thyroid hormones, 49% (4705) of which were children under the age of 5. Of the total number of reported exposures, there were minor or moderate symptoms in only 0.2% of patients, and there were no deaths.²

Pathophysiology

The thyroid gland produces triiodothyronine (T3) and tetraiodothyronine (T4), which are released into the systemic circulation. T3 and T4 secretion is regulated by the hypothalamic–pituitary–thyroid axis. The hypothalamus secretes thyrotropin releasing hormone (TRH) which reaches the anterior pituitary via the pituitary portal tract, inducing the systemic release of thyroid stimulating hormone (TSH). TSH stimulates the production and release of T3 and T4 (thyroxine) from the thyroid. T3 has three times the pharmacological activity as T4.³ Approximately 15% of circulating T3 is secreted by the thyroid gland; the balance is from the extrathyroid conversion of T4, primarily in the kidneys and liver. Thyroxine's activity is solely related to this conversion to T3. Available thyroid hormone supplements are levothyroxine sodium (T4) and liothyronine sodium (T3). Additional available thyroid hormone formulations include products containing both T3 and T4. These include bovine-desiccated thyroid and liotrix, a formulation containing a mixture of T4 and T3 in a 4:1 ratio.

Clinical Findings

An excess of thyroid hormone (hyperthyroidism) may result from excess thyroid production, pathological processes such as thyroid carcinoma, or inadvertent or intentional acute or chronic ingestion of thyroid supplements. The clinical manifestations of hyperthyroidism result from a hypermetabolic state mimicking adrenergic excess: tachycardia, anxiety, tremor, behavioral changes, and hyperthermia. These symptoms of thyroid excess are seen with chronic ingestion of thyroid hormone and are more ...