Thyroid Hormone Physiology and Pharmacokinetics

Thyrotropin-releasing hormone is secreted from the hypothalamus and stimulates the pituitary to release thyroid-stimulating hormone. Thyroid-stimulating hormone then acts on thyroid follicular cells to drive iodide trapping, thyroid hormone synthesis, and hormone secretion. When thyroid hormone levels rise sufficiently, they feed back on both the pituitary and hypothalamus to suppress further thyroid-stimulating hormone release. This closed-loop architecture is what makes thyroid-stimulating hormone such a sensitive monitor of thyroid hormone status.

Dopamine and somatostatin tonically inhibit thyroid-stimulating hormone secretion from the pituitary. This explains the mild secondary hypothyroidism occasionally seen with prolonged high-dose dopamine infusion and the transient thyroid-stimulating hormone suppression produced by somatostatin analogues such as octreotide.

Thyroid-stimulating hormone shares an alpha subunit with luteinizing hormone, follicle-stimulating hormone, and human chorionic gonadotropin. The clinical relevance: in the first trimester of pregnancy and in gestational trophoblastic disease, human chorionic gonadotropin reaches very high concentrations and cross-reacts with the thyroid-stimulating hormone receptor, suppressing thyroid-stimulating hormone while stimulating thyroid hormone output enough to cause gestational hyperthyroidism.

The pituitary is uniquely rich in type 2 deiodinase, which locally converts thyroxine to triiodothyronine. This makes thyroid-stimulating hormone suppression highly sensitive to circulating thyroxine levels, and it is why thyroid-stimulating hormone is the most sensitive single marker of levothyroxine adequacy. A persistently elevated thyroid-stimulating hormone on a stable dose almost always signals inadequate dosing or an absorption problem.

The thyroid-stimulating hormone–free thyroxine relationship is log-linear: a twofold change in free thyroxine produces roughly a 100-fold change in thyroid-stimulating hormone. This amplification makes thyroid-stimulating hormone a sensitive detector of small changes in thyroid hormone status — but also means that a thyroid-stimulating hormone of 8 mIU/L represents a much smaller hormone deficit than a thyroid-stimulating hormone of 80 mIU/L.

Type 1 deiodinase is expressed in the liver, kidney, thyroid, and skeletal muscle. It converts thyroxine to triiodothyronine in peripheral tissues and clears reverse triiodothyronine from circulation. Type 1 deiodinase is inhibited by propylthiouracil, amiodarone, and propranolol at high doses — this explains the peripheral triiodothyronine-lowering effect of propylthiouracil beyond its thyroid peroxidase inhibition, and the rise in reverse triiodothyronine seen with amiodarone.

Type 2 deiodinase is expressed in the pituitary, brain, and heart, and is the primary source of intracellular triiodothyronine in the brain. It is upregulated in hypothyroid states and downregulated when thyroxine concentrations rise, serving as a local homeostatic buffer. Because the pituitary uses type 2 deiodinase to generate its triiodothyronine signal, thyroid-stimulating hormone suppression tracks circulating thyroxine levels closely.

Type 3 deiodinase is expressed at high levels in the placenta and fetal tissues. It inactivates both thyroxine and triiodothyronine, protecting the fetus from premature exposure to physiologically active thyroid hormone during neurodevelopment.

In critically ill patients, type 3 deiodinase is upregulated in peripheral tissues while type 1 deiodinase activity falls. The result is low triiodothyronine, elevated reverse triiodothyronine, and low-normal thyroid-stimulating hormone — without any primary thyroid gland dysfunction. This pattern is called sick euthyroid syndrome or nonthyroidal illness syndrome.

Levothyroxine (synthetic thyroxine) is the standard preparation for thyroid hormone replacement. Oral bioavailability averages 70–80% from standard tablets and is substantially affected by gastric pH — absorption requires an acidic environment and occurs primarily in the proximal small intestine. The elimination half-life is 6–7 days, meaning four to five half-lives are needed to reach steady state after any dose change. Thyroid-stimulating hormone should not be rechecked sooner than six weeks after initiation or dose adjustment, because earlier values do not reflect steady-state drug concentrations.

Approximately 99.97% of circulating thyroxine is bound to thyroid-binding globulin, transthyretin, and albumin; only the free fraction (0.03%) is biologically active. Standard assays measure free thyroxine to avoid the confounding effect of altered binding protein concentrations. Pregnancy and estrogen therapy increase thyroid-binding globulin, raising total thyroxine while free thyroxine and thyroid-stimulating hormone remain unchanged in euthyroid individuals.

Liothyronine (synthetic triiodothyronine) has a much shorter half-life of approximately 1 day and higher bioavailability than levothyroxine, producing larger peak-to-trough fluctuations in serum triiodothyronine. Its primary clinical use is in hypothyroid preparation for radioactive iodine scanning — it can be stopped 2 weeks before scanning rather than the 4 weeks required for levothyroxine withdrawal, minimizing hypothyroid symptom burden. It is also used in myxedema coma for rapid triiodothyronine repletion and, controversially, in combination with levothyroxine for patients who remain symptomatic on levothyroxine monotherapy.