Abstract
Background:
We upgraded our recent feedback control system (FBCS) simulation model of human thyroid hormone (TH) regulation to include explicit representation of hypothalamic and pituitary dynamics, and updated TH distribution and elimination (D&E) parameters. This new model greatly expands the range of clinical and basic science scenarios explorable by computer simulation.
Methods:
We quantified the model from pharmacokinetic (PK) and physiological human data and validated it comparatively against several independent clinical data sets. We then explored three contemporary clinical issues with the new model: combined triiodothyronine (T3)/thyroxine (T4) versus T4-only treatment, parenteral levothyroxine (L-T4) administration, and central hypothyroidism.
Results:
Combined T3/T4 therapy—In thyroidectomized patients, the L-T4–only replacement doses needed to normalize plasma T3 or average tissue T3 were 145 μg L-T4/day or 165 μg L-T4/day, respectively. The combined T4 + T3 dosing needed to normalize both plasma and tissue T3 levels was 105 μg L-T4 + 9 μg T3 per day. For all three regimens, simulated mean steady-state plasma thyroid-stimulating hormone (TSH), T3, and T4 was within normal ranges (TSH: 0.5–5 mU/L; T4: 5–12 μg/dL; T3: 0.8–1.9 ng/mL). Parenteral T4 administration—800 μg weekly or 400 μg twice weekly normalized average tissue T3 levels both for subcutaneous (SC) and intramuscular (IM) routes of administration. TSH, T3, and T4 levels were maintained within normal ranges for all four of these dosing schemes (1× vs. 2× weekly, SC vs. IM). Central hypothyroidism—We simulated steady-state plasma T3, T4, and TSH concentrations in response to varying degrees of central hypothyroidism, reducing TSH secretion from 50% down to 0.1% of normal. Surprisingly, TSH, T3, and T4 plasma concentrations remained within normal ranges for TSH secretion as low as 25% of normal.
Conclusions:
Combined T3/T4 treatment—Simulated standard L-T4–only therapy was sufficient to renormalize average tissue T3 levels and maintain normal TSH, T3, and T4 plasma levels, supporting adequacy of standard L-T4–only treatment. Parenteral T4 administration—TSH, T3, and T4 levels were maintained within normal ranges for all four of these dosing schemes (1× vs. 2× weekly, SC vs. IM), supporting these therapeutic alternatives for patients with compromised L-T4 gut absorption. Central hypothyroidism—These results highlight how highly nonlinear feedback in the hypothalamic-pituitary-thyroid axis acts to maintain normal hormone levels, even with severely reduced TSH secretion.